WordPress Performance Review Skill

Overview

Systematic performance code review for WordPress themes, plugins, and custom code. Core principle: Scan critical issues first (OOM, unbounded queries, cache bypass), then warnings, then optimizations. Report with line numbers and severity levels.

When to Use

Use when:

• Reviewing PR/code for WordPress theme or plugin
• User reports slow page loads, timeouts, or 500 errors
• Auditing before high-traffic event (launch, sale, viral moment)
• Optimizing WP_Query or database operations
• Investigating memory exhaustion or DB locks

Don't use for:

• Security-only audits (use wp-security-review when available)
• Gutenberg block development patterns (use wp-gutenberg-blocks when available)
• General PHP code review not specific to WordPress

Code Review Workflow

1. Identify file type and apply relevant checks below
2. Scan for critical patterns first (OOM, unbounded queries, cache bypass)
3. Check warnings (inefficient but not catastrophic)
4. Note optimizations (nice-to-have improvements)
5. Report with line numbers using output format below

File-Type Specific Checks

Plugin/Theme PHP Files (functions.php, plugin.php, *.php)

Scan for:

• query_posts() → CRITICAL: Never use - breaks main query
• posts_per_page.*-1 or numberposts.*-1 → CRITICAL: Unbounded query
• session_start() → CRITICAL: Bypasses page cache
• add_action.*init.* or add_action.*wp_loaded → Check if expensive code runs every request
• update_option or add_option in non-admin context → WARNING: DB writes on page load
• wp_remote_get or wp_remote_post without caching → WARNING: Blocking HTTP

WP_Query / Database Code

Scan for:

• Missing posts_per_page argument → WARNING: Defaults to blog setting
• 'meta_query' with 'value' comparisons → WARNING: Unindexed column scan
• post__not_in with large arrays → WARNING: Slow exclusion
• LIKE '%term%' (leading wildcard) → WARNING: Full table scan
• Missing no_found_rows => true when not paginating → INFO: Unnecessary count

AJAX Handlers (wp_ajax_*, REST endpoints)

Scan for:

• admin-ajax.php usage → INFO: Consider REST API instead
• POST method for read operations → WARNING: Bypasses cache
• setInterval or polling patterns → CRITICAL: Self-DDoS risk
• Missing nonce verification → Security issue (not performance, but flag it)

Template Files (*.php in theme)

Scan for:

• get_template_part in loops → WARNING: Consider caching output
• Database queries inside loops (N+1) → CRITICAL: Query multiplication
• wp_remote_get in templates → WARNING: Blocks rendering

JavaScript Files

Scan for:

• $.post( for read operations → WARNING: Use GET for cacheability
• setInterval.*fetch\|ajax → CRITICAL: Polling pattern
• import _ from 'lodash' → WARNING: Full library import bloats bundle
• Inline <script> making AJAX calls on load → Check necessity

Block Editor / Gutenberg Files (block.json, *.js in blocks/)

Scan for:

• Many registerBlockStyle() calls → WARNING: Each creates preview iframe
• wp_kses_post($content) in render callbacks → WARNING: Breaks InnerBlocks
• Static blocks without render_callback → INFO: Consider dynamic for maintainability

Asset Registration (functions.php, *.php)

Scan for:

• wp_enqueue_script without version → INFO: Cache busting issues
• wp_enqueue_script without defer/async strategy → INFO: Blocks rendering
• Missing THEME_VERSION constant → INFO: Version management
• wp_enqueue_script without conditional check → WARNING: Assets load globally when only needed on specific pages

Transients & Options

Scan for:

• set_transient with dynamic keys (e.g., user_{$id}) → WARNING: Table bloat without object cache
• set_transient for frequently-changing data → WARNING: Defeats caching purpose
• Large data in transients on shared hosting → WARNING: DB bloat without object cache

WP-Cron

Scan for:

• Missing DISABLE_WP_CRON constant → INFO: Cron runs on page requests
• Long-running cron callbacks (loops over all users/posts) → CRITICAL: Blocks cron queue
• wp_schedule_event without checking if already scheduled → WARNING: Duplicate schedules

Search Patterns for Quick Detection

# Critical issues - scan these first
grep -rn "posts_per_page.*-1\|numberposts.*-1" .
grep -rn "query_posts\s*(" .
grep -rn "session_start\s*(" .
grep -rn "setInterval.*fetch\|setInterval.*ajax\|setInterval.*\\\$\." .

# Database writes on frontend
grep -rn "update_option\|add_option" . | grep -v "admin\|activate\|install"

# Uncached expensive functions
grep -rn "url_to_postid\|attachment_url_to_postid\|count_user_posts" .

# External HTTP without caching
grep -rn "wp_remote_get\|wp_remote_post\|file_get_contents.*http" .

# Cache bypass risks
grep -rn "setcookie\|session_start" .

# PHP code anti-patterns
grep -rn "in_array\s*(" . | grep -v "true\s*)" # Missing strict comparison
grep -rn "<<<" .  # Heredoc/nowdoc syntax
grep -rn "cache_results.*false" .

# JavaScript bundle issues
grep -rn "import.*from.*lodash['\"]" .  # Full lodash import
grep -rn "registerBlockStyle" .  # Many block styles = performance issue

# Asset loading issues
grep -rn "wp_enqueue_script\|wp_enqueue_style" . | grep -v "is_page\|is_singular\|is_admin"

# Transient misuse
grep -rn "set_transient.*\\\$" .  # Dynamic transient keys
grep -rn "set_transient" . | grep -v "get_transient"  # Set without checking first

# WP-Cron issues
grep -rn "wp_schedule_event" . | grep -v "wp_next_scheduled"  # Missing schedule check

Platform Context

Different hosting environments require different approaches:

Managed WordPress Hosts (WP Engine, Pantheon, Pressable, WordPress VIP, etc.):

• Often provide object caching out of the box
• May have platform-specific helper functions (e.g., wpcom_vip_* on VIP)
• Check host documentation for recommended patterns

Self-Hosted / Standard Hosting:

• Implement object caching wrappers manually for expensive functions
• Consider Redis or Memcached plugins for persistent object cache
• More responsibility for caching layer configuration

Shared Hosting:

• Be extra cautious about unbounded queries and external HTTP
• Limited resources mean performance issues surface faster
• May lack persistent object cache entirely

Quick Reference: Critical Anti-Patterns

Database Queries

// ❌ CRITICAL: Unbounded query.
'posts_per_page' => -1

// ✅ GOOD: Set reasonable limit, paginate if needed.
'posts_per_page' => 100,
'no_found_rows'  => true, // Skip count if not paginating.

// ❌ CRITICAL: Never use query_posts().
query_posts( 'cat=1' ); // Breaks pagination, conditionals.

// ✅ GOOD: Use WP_Query or pre_get_posts filter.
$query = new WP_Query( array( 'cat' => 1 ) );
// Or modify main query:
add_action( 'pre_get_posts', function( $query ) {
    if ( $query->is_main_query() && ! is_admin() ) {
        $query->set( 'cat', 1 );
    }
} );

// ❌ CRITICAL: Missing WHERE clause (falsy ID becomes 0).
$query = new WP_Query( array( 'p' => intval( $maybe_false_id ) ) );

// ✅ GOOD: Validate ID before querying.
if ( ! empty( $maybe_false_id ) ) {
    $query = new WP_Query( array( 'p' => intval( $maybe_false_id ) ) );
}

// ❌ WARNING: LIKE with leading wildcard (full table scan).
$wpdb->get_results( "SELECT * FROM wp_posts WHERE post_title LIKE '%term%'" );

// ✅ GOOD: Use trailing wildcard only, or use WP_Query 's' parameter.
$wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM wp_posts WHERE post_title LIKE %s",
    $wpdb->esc_like( $term ) . '%'
) );

// ❌ WARNING: NOT IN queries (filter in PHP instead).
'post__not_in' => $excluded_ids

// ✅ GOOD: Fetch all, filter in PHP (faster for large exclusion lists).
$posts = get_posts( array( 'posts_per_page' => 100 ) );
$posts = array_filter( $posts, function( $post ) use ( $excluded_ids ) {
    return ! in_array( $post->ID, $excluded_ids, true );
} );

Hooks & Actions

// ❌ WARNING: Code runs on every request via init.
add_action( 'init', 'expensive_function' );

// ✅ GOOD: Check context before running expensive code.
add_action( 'init', function() {
    if ( is_admin() || wp_doing_cron() ) {
        return;
    }
    // Frontend-only code here.
} );

// ❌ CRITICAL: Database writes on every page load.
add_action( 'wp_head', 'prefix_bad_tracking' );
function prefix_bad_tracking() {
    update_option( 'last_visit', time() );
}

// ✅ GOOD: Use object cache buffer, flush via cron.
add_action( 'shutdown', function() {
    wp_cache_incr( 'page_views_buffer', 1, 'counters' );
} );

// ❌ WARNING: Using admin-ajax.php instead of REST API.
// Prefer: register_rest_route() - leaner bootstrap.

PHP Code

// ❌ WARNING: O(n) lookup - use isset() with associative array.
in_array( $value, $array ); // Also missing strict = true.

// ✅ GOOD: O(1) lookup with isset().
$allowed = array( 'foo' => true, 'bar' => true );
if ( isset( $allowed[ $value ] ) ) {
    // Process.
}

// ❌ WARNING: Heredoc prevents late escaping.
$html = <<<HTML
<div>$unescaped_content</div>
HTML;

// ✅ GOOD: Escape at output.
printf( '<div>%s</div>', esc_html( $content ) );

Caching Issues

// ❌ WARNING: Uncached expensive function calls.
url_to_postid( $url );
attachment_url_to_postid( $attachment_url );
count_user_posts( $user_id );
wp_oembed_get( $url );

// ✅ GOOD: Wrap with object cache (works on any host).
function prefix_cached_url_to_postid( $url ) {
    $cache_key = 'url_to_postid_' . md5( $url );
    $post_id   = wp_cache_get( $cache_key, 'url_lookups' );

    if ( false === $post_id ) {
        $post_id = url_to_postid( $url );
        wp_cache_set( $cache_key, $post_id, 'url_lookups', HOUR_IN_SECONDS );
    }

    return $post_id;
}

// ✅ GOOD: On WordPress VIP, use platform helpers instead.
// wpcom_vip_url_to_postid(), wpcom_vip_attachment_url_to_postid(), etc.

// ❌ WARNING: Large autoloaded options.
add_option( 'prefix_large_data', $data ); // Add: , '', 'no' for autoload.

// ❌ INFO: Missing wp_cache_get_multiple for batch lookups.
foreach ( $ids as $id ) {
    wp_cache_get( "key_{$id}" );
}

AJAX & External Requests

// ❌ WARNING: AJAX POST request (bypasses cache).
$.post( ajaxurl, data ); // Prefer: $.get() for read operations.

// ❌ CRITICAL: Polling pattern (self-DDoS).
setInterval( () => fetch( '/wp-json/...' ), 5000 );

// ❌ WARNING: Synchronous external HTTP in page load.
wp_remote_get( $url ); // Cache result or move to cron.

// ✅ GOOD: Set timeout and handle errors.
$response = wp_remote_get( $url, array( 'timeout' => 2 ) );
if ( is_wp_error( $response ) ) {
    return get_fallback_data();
}

WP Cron

// ❌ WARNING: WP Cron runs on page requests.
// Add to wp-config.php:
define( 'DISABLE_WP_CRON', true );
// Run via server cron: * * * * * wp cron event run --due-now

// ❌ CRITICAL: Long-running cron blocks entire queue.
add_action( 'my_daily_sync', function() {
    foreach ( get_users() as $user ) { // 50k users = hours.
        sync_user_data( $user );
    }
} );

// ✅ GOOD: Batch processing with rescheduling.
add_action( 'my_batch_sync', function() {
    $offset = (int) get_option( 'sync_offset', 0 );
    $users  = get_users( array( 'number' => 100, 'offset' => $offset ) );

    if ( empty( $users ) ) {
        delete_option( 'sync_offset' );
        return;
    }

    foreach ( $users as $user ) {
        sync_user_data( $user );
    }

    update_option( 'sync_offset', $offset + 100 );
    wp_schedule_single_event( time() + 60, 'my_batch_sync' );
} );

// ❌ WARNING: Scheduling without checking if already scheduled.
wp_schedule_event( time(), 'hourly', 'my_task' ); // Creates duplicates!

// ✅ GOOD: Check before scheduling.
if ( ! wp_next_scheduled( 'my_task' ) ) {
    wp_schedule_event( time(), 'hourly', 'my_task' );
}

Cache Bypass Issues

// ❌ CRITICAL: Plugin starts PHP session on frontend (bypasses ALL page cache).
session_start(); // Check plugins for this - entire site becomes uncacheable!

// ❌ WARNING: Unique query params create cache misses.
// https://example.com/?utm_source=fb&utm_campaign=123&fbclid=abc
// Each unique URL = separate cache entry = cache miss.
// Solution: Strip marketing params at CDN/edge level.

// ❌ WARNING: Setting cookies on public pages.
setcookie( 'visitor_id', $id ); // Prevents caching for that user.

Transients Misuse

// ❌ WARNING: Dynamic transient keys create table bloat (without object cache).
set_transient( "user_{$user_id}_cart", $data, HOUR_IN_SECONDS );
// 10,000 users = 10,000 rows in wp_options!

// ✅ GOOD: Use object cache for user-specific data.
wp_cache_set( "cart_{$user_id}", $data, 'user_carts', HOUR_IN_SECONDS );

// ❌ WARNING: Transients for frequently-changing data defeats purpose.
set_transient( 'visitor_count', $count, 60 ); // Changes every minute.

// ✅ GOOD: Use object cache for volatile data.
wp_cache_set( 'visitor_count', $count, 'stats' );

// ❌ WARNING: Large data in transients on shared hosting.
set_transient( 'api_response', $megabytes_of_json, DAY_IN_SECONDS );
// Without object cache = serialized blob in wp_options.

// ✅ GOOD: Check hosting before using transients for large data.
if ( wp_using_ext_object_cache() ) {
    set_transient( 'api_response', $data, DAY_IN_SECONDS );
} else {
    // Store in files or skip caching on shared hosting.
}

Asset Loading

// ❌ WARNING: Assets load globally when only needed on specific pages.
add_action( 'wp_enqueue_scripts', function() {
    wp_enqueue_script( 'contact-form-js', ... );
    wp_enqueue_style( 'contact-form-css', ... );
} );

// ✅ GOOD: Conditional enqueue based on page/template.
add_action( 'wp_enqueue_scripts', function() {
    if ( is_page( 'contact' ) || is_page_template( 'contact-template.php' ) ) {
        wp_enqueue_script( 'contact-form-js', ... );
        wp_enqueue_style( 'contact-form-css', ... );
    }
} );

// ✅ GOOD: Only load WooCommerce assets on shop pages.
add_action( 'wp_enqueue_scripts', function() {
    if ( ! is_woocommerce() && ! is_cart() && ! is_checkout() ) {
        wp_dequeue_style( 'woocommerce-general' );
        wp_dequeue_script( 'wc-cart-fragments' );
    }
} );

External API Requests

// ❌ WARNING: No timeout set (default is 5 seconds).
wp_remote_get( $url ); // Set timeout: array( 'timeout' => 2 ).

// ❌ WARNING: Missing error handling for API failures.
$response = wp_remote_get( $url );
echo $response['body']; // Check is_wp_error() first!

Sitemaps & Redirects

// ❌ WARNING: Generating sitemaps for deep archives (crawlers hammer these).
// Solution: Exclude old post types, cache generated sitemaps.

// ❌ CRITICAL: Redirect loops consuming CPU.
// Debug with: x-redirect-by header, wp_debug_backtrace_summary().

Post Meta Queries

// ❌ WARNING: Searching meta_value without index.
'meta_query' => array(
    array(
        'key'   => 'color',
        'value' => 'red',
    ),
)
// Better: Use taxonomy or encode value in meta_key name.

// ❌ WARNING: Binary meta values requiring value scan.
'meta_key'   => 'featured',
'meta_value' => 'true',
// Better: Presence of 'is_featured' key = true, absence = false.

For deeper context on any pattern: Load references/anti-patterns.md

Severity Definitions

Severity	Description
Critical	Will cause failures at scale (OOM, 500 errors, DB locks)
Warning	Degrades performance under load
Info	Optimization opportunity

Output Format

Structure findings as:

## Performance Review: [filename/component]

### Critical Issues
- **Line X**: [Issue] - [Explanation] - [Fix]

### Warnings  
- **Line X**: [Issue] - [Explanation] - [Fix]

### Recommendations
- [Optimization opportunities]

### Summary
- Total issues: X Critical, Y Warnings, Z Info
- Estimated impact: [High/Medium/Low]

Common Mistakes

When performing performance reviews, avoid these errors:

Mistake	Why It's Wrong	Fix
Flagging posts_per_page => -1 in admin-only code	Admin queries don't face public scale	Check context - admin, CLI, cron are lower risk
Missing the session_start() buried in a plugin	Cache bypass affects entire site	Always grep for session_start across all code
Ignoring no_found_rows for non-paginated queries	Small optimization but adds up	Flag as INFO, not WARNING
Recommending object cache on shared hosting	Many shared hosts lack persistent cache	Check hosting environment first
Only reviewing PHP, missing JS polling	JS setInterval + fetch = self-DDoS	Review .js files for polling patterns

Deep-Dive References

Load these references based on the task:

Task	Reference to Load
Reviewing PHP code for issues	references/anti-patterns.md
Optimizing WP_Query calls	references/wp-query-guide.md
Implementing caching	references/caching-guide.md
High-traffic event prep	references/measurement-guide.md

Note: For standard code reviews, anti-patterns.md contains all patterns needed. Other references provide deeper context when specifically optimizing queries, implementing caching strategies, or preparing for traffic events.