WordPress provides several constants to control memory usage, caching, cron behavior, and script handling for optimal performance.

Memory Limits

WP_MEMORY_LIMIT

define( 'WP_MEMORY_LIMIT', '256M' );

• Type: String
• Default: 40M (single site), 64M (multisite)
• Purpose: Maximum memory WordPress can use for frontend operations
• Format: Number followed by M (megabytes) or G (gigabytes)
• Note: Cannot exceed PHP’s memory_limit setting

Common Values

WP_MAX_MEMORY_LIMIT

define( 'WP_MAX_MEMORY_LIMIT', '512M' );

• Type: String
• Default: 256M
• Purpose: Maximum memory for admin-side operations
• Use Case: Image uploads, imports, plugin operations
• Note: Admin tasks often need more memory than frontend

Memory Limit Example

// Frontend: 256MB, Admin: 512MB
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );

Checking PHP Memory Limit

php -i | grep memory_limit

WordPress can only use up to PHP’s limit:

// If PHP limit is 128M, this won't work
define( 'WP_MEMORY_LIMIT', '256M' ); // Silently limited to 128M

Object Caching

WP_CACHE

define( 'WP_CACHE', true );

• Type: Boolean
• Default: false
• Purpose: Enables advanced caching functionality
• Effect: Loads wp-content/advanced-cache.php if it exists
• Note: Typically set by caching plugins (W3 Total Cache, WP Super Cache, etc.)

How It Works

1. WordPress checks for WP_CACHE constant
2. If true, loads wp-content/advanced-cache.php
3. Caching plugin’s drop-in handles page caching
4. Cached content served without full WordPress load

Manual advanced-cache.php

<?php
// wp-content/advanced-cache.php
// Custom caching logic
$cache_file = WP_CONTENT_DIR . '/cache/' . md5( $_SERVER['REQUEST_URI'] ) . '.html';

if ( file_exists( $cache_file ) && ! is_user_logged_in() ) {
    readfile( $cache_file );
    exit;
}

Object Cache Drop-In

For persistent object caching (Redis, Memcached):

// wp-content/object-cache.php
// Provided by Redis Object Cache or similar plugins

Common object cache plugins:

• Redis Object Cache
• Memcached Object Cache
• LiteSpeed Cache

Cron Settings

WordPress uses a pseudo-cron system triggered by page visits.

DISABLE_WP_CRON

define( 'DISABLE_WP_CRON', true );

• Type: Boolean
• Default: false
• Purpose: Disables WordPress’s built-in cron system
• Use Case: Replace with real system cron for better reliability

Why Disable WP-Cron?

1. Performance: Each page load checks for due cron jobs
2. Reliability: Low-traffic sites may miss scheduled tasks
3. Control: System cron runs on exact schedule
4. Timeouts: Long cron jobs don’t affect page loads

Setting Up System Cron

Linux crontab:

# Run WordPress cron every 5 minutes
*/5 * * * * curl -s https://example.com/wp-cron.php?doing_wp_cron > /dev/null 2>&1

# Or using wget
*/5 * * * * wget -q -O - https://example.com/wp-cron.php?doing_wp_cron > /dev/null 2>&1

# Or using WP-CLI
*/5 * * * * cd /var/www/html && wp cron event run --due-now > /dev/null 2>&1

ALTERNATE_WP_CRON

define( 'ALTERNATE_WP_CRON', true );

• Type: Boolean
• Default: false
• Purpose: Uses alternate cron method via redirect
• Use Case: When standard cron fails due to server configuration
• Effect: Fires cron via JavaScript redirect instead of background request

WP_CRON_LOCK_TIMEOUT

define( 'WP_CRON_LOCK_TIMEOUT', 60 );

• Type: Integer (seconds)
• Default: 60
• Purpose: Minimum time between cron process spawns
• Effect: Prevents multiple cron processes from overlapping

Script and Style Optimization

CONCATENATE_SCRIPTS

define( 'CONCATENATE_SCRIPTS', true );

• Type: Boolean
• Default: true (admin), varies (frontend)
• Purpose: Combines admin JavaScript files into fewer HTTP requests
• Effect When True: Reduces HTTP requests in wp-admin
• Note: Disabled automatically when SCRIPT_DEBUG is true

COMPRESS_SCRIPTS

define( 'COMPRESS_SCRIPTS', true );

• Type: Boolean
• Default: true
• Purpose: Compresses JavaScript output using gzip
• Effect: Smaller script payloads

COMPRESS_CSS

define( 'COMPRESS_CSS', true );

• Type: Boolean
• Default: true
• Purpose: Compresses CSS output using gzip
• Effect: Smaller stylesheet payloads

ENFORCE_GZIP

define( 'ENFORCE_GZIP', true );

• Type: Boolean
• Default: false
• Purpose: Forces gzip encoding when outputting compressed scripts

Performance Optimization Example

// Production performance settings
define( 'CONCATENATE_SCRIPTS', true );
define( 'COMPRESS_SCRIPTS', true );
define( 'COMPRESS_CSS', true );
define( 'ENFORCE_GZIP', true );

// Disable emoji script
define( 'DISABLE_WP_EMOJI', true ); // Custom - not core, but commonly added

Content Settings Affecting Performance

WP_POST_REVISIONS

define( 'WP_POST_REVISIONS', 5 );

• Type: Boolean or Integer
• Default: true (unlimited revisions)
• Purpose: Controls post revision storage
• Values:
  • true – Unlimited revisions (default)
  • false – Disable revisions entirely
  • Integer – Maximum number of revisions to keep

Revision Settings

AUTOSAVE_INTERVAL

define( 'AUTOSAVE_INTERVAL', 300 );

• true – Unlimited revisions (default)
• false – Disable revisions entirely
• Integer – Maximum number of revisions to keep

EMPTY_TRASH_DAYS

define( 'EMPTY_TRASH_DAYS', 7 );

• true – Unlimited revisions (default)
• false – Disable revisions entirely
• Integer – Maximum number of revisions to keep

Media Settings

IMAGE_EDIT_OVERWRITE

define( 'IMAGE_EDIT_OVERWRITE', true );

• Type: Integer (seconds)
• Default: 60 (1 minute)
• Purpose: Interval between automatic post saves
• Effect: Reduces server requests when editing
• Recommended: 120-300 seconds for better performance

Database Query Optimization

DO_NOT_UPGRADE_GLOBAL_TABLES

define( 'DO_NOT_UPGRADE_GLOBAL_TABLES', true );

• Type: Integer (days)
• Default: 30
• Purpose: Days before trashed items are permanently deleted
• Values:
  • 0 – Disable trash (items deleted immediately)
  • 7 – Empty trash weekly
  • 30 – Default monthly cleanup

External Request Optimization

WP_HTTP_BLOCK_EXTERNAL

define( 'WP_HTTP_BLOCK_EXTERNAL', true );

• 0 – Disable trash (items deleted immediately)
• 7 – Empty trash weekly
• 30 – Default monthly cleanup

WP_ACCESSIBLE_HOSTS

define( 'WP_ACCESSIBLE_HOSTS', 'api.wordpress.org,*.github.com' );

• 0 – Disable trash (items deleted immediately)
• 7 – Empty trash weekly
• 30 – Default monthly cleanup

Filesystem Method

FS_METHOD

define( 'FS_METHOD', 'direct' );

• Type: Boolean
• Default: false
• Purpose: Overwrites original images when editing instead of creating copies
• Effect: Saves disk space, but loses original

FS_CHMOD_DIR / FS_CHMOD_FILE

define( 'FS_CHMOD_DIR', 0755 );
define( 'FS_CHMOD_FILE', 0644 );

• Type: Boolean
• Default: Not defined
• Purpose: Prevents upgrades of global tables in multisite
• Use Case: Shared user tables across networks

Complete Performance Configuration

High-Traffic Production Site

<?php
// Memory
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );

// Caching (managed by caching plugin)
define( 'WP_CACHE', true );

// Cron - use system cron
define( 'DISABLE_WP_CRON', true );

// Script optimization
define( 'CONCATENATE_SCRIPTS', true );
define( 'COMPRESS_SCRIPTS', true );
define( 'COMPRESS_CSS', true );
define( 'ENFORCE_GZIP', true );

// Content limits
define( 'WP_POST_REVISIONS', 5 );
define( 'AUTOSAVE_INTERVAL', 300 );
define( 'EMPTY_TRASH_DAYS', 7 );

// Filesystem
define( 'FS_METHOD', 'direct' );

Development Environment

<?php
// Memory (generous for debugging)
define( 'WP_MEMORY_LIMIT', '512M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );

// No caching
define( 'WP_CACHE', false );

// Keep WP-Cron for testing
// don't define DISABLE_WP_CRON

// Uncompressed for debugging
define( 'CONCATENATE_SCRIPTS', false );
define( 'COMPRESS_SCRIPTS', false );
define( 'COMPRESS_CSS', false );

// Full revisions for development
define( 'WP_POST_REVISIONS', true );
define( 'AUTOSAVE_INTERVAL', 60 );

Low-Resource Environment

<?php
// Conservative memory
define( 'WP_MEMORY_LIMIT', '64M' );
define( 'WP_MAX_MEMORY_LIMIT', '128M' );

// Minimal revisions
define( 'WP_POST_REVISIONS', 2 );
define( 'AUTOSAVE_INTERVAL', 600 );
define( 'EMPTY_TRASH_DAYS', 3 );

// Block external requests except essentials
define( 'WP_HTTP_BLOCK_EXTERNAL', true );
define( 'WP_ACCESSIBLE_HOSTS', 'api.wordpress.org,downloads.wordpress.org' );

Monitoring Performance

Query Count

// Add to theme footer
echo get_num_queries() . ' queries in ' . timer_stop() . ' seconds';

Memory Usage

echo 'Memory: ' . round( memory_get_peak_usage() / 1024 / 1024, 2 ) . 'MB';

Recommended Tools

1. Query Monitor – Detailed performance analysis
2. New Relic – Application performance monitoring
3. Debug Bar – Admin toolbar debugging
4. Server Timing – Browser developer tools