WordPress Performance

Performance optimization patterns for WordPress plugins, themes, and live sites.

Critical Rules

1. Never query without limits — posts_per_page => -1 and unbounded $wpdb queries are the most common cause of memory exhaustion on production sites.
2. Cache expensive queries — any query that runs on every page load and doesn't change per-request must be cached with transients or the object cache.
3. Use no_found_rows — set 'no_found_rows' => true on any WP_Query that doesn't need pagination; this skips SQL_CALC_FOUND_ROWS which is expensive on large tables.
4. Avoid meta queries on unindexed keys — meta_query on wp_postmeta performs full table scans unless you add custom indexes. Consider a custom table for heavily queried data.
5. Defer heavy work — operations on init run on every request. Use admin_init for admin-only work, rest_api_init for REST-only, and wp_loaded or later hooks when possible.
6. Prime caches, don't loop-query — use update_post_meta_cache(), update_post_caches(), or _prime_post_caches() to batch-load metadata instead of calling get_post_meta() in a loop.

Query Performance Patterns

Efficient WP_Query

// Good — limited, cache-friendly, no unnecessary data:
$query = new WP_Query( [
    'post_type'              => 'product',
    'posts_per_page'         => 20,
    'no_found_rows'          => true,   // Skip pagination count query.
    'update_post_meta_cache' => false,  // Skip if not reading meta.
    'update_post_term_cache' => false,  // Skip if not reading terms.
    'fields'                 => 'ids',  // Return only IDs when full objects aren't needed.
] );

// Bad — unbounded, forces full table scan:
$query = new WP_Query( [
    'post_type'      => 'product',
    'posts_per_page' => -1,  // Never do this.
] );

N+1 Query Prevention

// Bad — N+1 pattern (1 query per post in the loop):
foreach ( $posts as $post ) {
    $price = get_post_meta( $post->ID, '_price', true );  // Query per iteration.
}

// Good — prime the cache first, then loop reads from cache:
$post_ids = wp_list_pluck( $posts, 'ID' );
update_meta_cache( 'post', $post_ids );  // Single query loads all meta.

foreach ( $posts as $post ) {
    $price = get_post_meta( $post->ID, '_price', true );  // Reads from cache.
}

Batch Operations

// Bad — individual inserts in a loop:
foreach ( $items as $item ) {
    $wpdb->insert( $table, $item );  // N queries.
}

// Good — single bulk insert:
$values = [];
foreach ( $items as $item ) {
    $values[] = $wpdb->prepare( '(%d, %s, %f)', $item['user_id'], $item['status'], $item['total'] );
}
if ( $values ) {
    $wpdb->query( "INSERT INTO {$table} (user_id, status, total) VALUES " . implode( ', ', $values ) );
}

Caching Strategies

Transient Caching Pattern

function myplugin_get_featured_products(): array {
    $cache_key = 'myplugin_featured_products';
    $data      = get_transient( $cache_key );

    if ( false !== $data ) {
        return $data;
    }

    $query = new WP_Query( [
        'post_type'      => 'product',
        'posts_per_page' => 12,
        'meta_key'       => '_featured',
        'meta_value'     => 'yes',
        'no_found_rows'  => true,
        'fields'         => 'ids',
    ] );

    $data = $query->posts;
    set_transient( $cache_key, $data, HOUR_IN_SECONDS );

    return $data;
}

// Invalidate when products change:
add_action( 'save_post_product', function (): void {
    delete_transient( 'myplugin_featured_products' );
} );

Object Cache for Per-Request Deduplication

function myplugin_get_settings(): array {
    $cached = wp_cache_get( 'settings', 'myplugin' );
    if ( false !== $cached ) {
        return $cached;
    }

    $settings = get_option( 'myplugin_settings', [] );
    wp_cache_set( 'settings', $settings, 'myplugin' );

    return $settings;
}

When to Use Each Layer

Hook Performance

Defer Initialization

// Bad — runs on every request including REST, AJAX, cron:
add_action( 'init', 'myplugin_heavy_init' );

// Good — scope to context:
add_action( 'admin_init', 'myplugin_admin_setup' );      // Admin only.
add_action( 'rest_api_init', 'myplugin_register_routes' ); // REST only.
add_action( 'template_redirect', 'myplugin_frontend' );    // Frontend only.

// Good — conditional loading:
add_action( 'init', function (): void {
    if ( ! is_admin() && ! wp_doing_ajax() && ! wp_doing_cron() ) {
        // Frontend-only logic.
    }
} );

Lazy Loading

// Bad — always loads all classes:
require_once __DIR__ . '/includes/class-admin.php';
require_once __DIR__ . '/includes/class-reports.php';
require_once __DIR__ . '/includes/class-import.php';

// Good — load only when needed:
add_action( 'admin_menu', function (): void {
    require_once __DIR__ . '/includes/class-admin.php';
    new Myplugin_Admin();
} );

// Best — PSR-4 autoloader via Composer:
require_once __DIR__ . '/vendor/autoload.php';

Autoload Optimization

Options with autoload = yes are loaded into memory on every page load via a single SELECT from wp_options. Large or rarely-used options bloat this query.

// Bad — large data autoloaded:
add_option( 'myplugin_log_history', $huge_array );  // Defaults to autoload = yes.

// Good — disable autoload for large or infrequent data:
add_option( 'myplugin_log_history', $huge_array, '', false );

// Or with update_option (autoload param is 4th arg since WP 4.2):
update_option( 'myplugin_log_history', $huge_array, false );

Rule of thumb: autoload only small, frequently-accessed options (settings, feature flags). Disable autoload for logs, large serialized arrays, and data accessed only on specific pages.

Database Indexing

// Add indexes to custom tables on activation:
register_activation_hook( __FILE__, function (): void {
    global $wpdb;
    $table = $wpdb->prefix . 'myplugin_orders';

    // Check before adding to make activation idempotent.
    $index_exists = $wpdb->get_var( $wpdb->prepare(
        'SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA = %s AND TABLE_NAME = %s AND INDEX_NAME = %s',
        DB_NAME, $table, 'status_created_idx'
    ) );

    if ( ! $index_exists ) {
        $wpdb->query( "ALTER TABLE {$table} ADD INDEX status_created_idx (status, created_at)" );
    }
} );

Indexing Post Meta for Queries

If you frequently filter by a specific meta key, consider adding an index:

-- Only add this if meta_query on this key is a known bottleneck:
ALTER TABLE wp_postmeta ADD INDEX meta_key_value_idx (meta_key(50), meta_value(50));

Warning: modifying core tables is risky and may conflict with updates. Prefer custom tables for heavily queried structured data.

Anti-Patterns

For query analysis and EXPLAIN usage, see reference/query-optimization.md. For caching architecture and invalidation strategies, see reference/caching-strategies.md.