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Troubleshooting WP-Optimize Cache Preload Issues: Common Problems and Solutions

25 threads Sep 11, 2025

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WP-Optimize's cache preload feature is designed to automatically build your page cache, ensuring visitors always get a fast, cached version of your site. However, users often encounter problems where preloading doesn't work as expected. Based on community reports, here are the most common preload issues and how to resolve them.

1. Preload Starts but Doesn't Finish or Populate Cache

Symptoms: Clicking "Run Now" initiates preloading, but the cache remains empty. The process may stop after 20-30 URLs. Server CPU might spike briefly, but no cache files are created in wp-content/cache/wpo-cache/.

Why it happens: This is frequently caused by conflicts with other caching layers or server configurations that interrupt the WordPress cron system, which the preload relies on to process tasks in chunks.

Solutions:

  • Check for conflicting caching plugins: Ensure no other page caching plugins (e.g., LiteSpeed Cache, W3 Total Cache) are active simultaneously. Server-level caching (e.g., Varnish, NGINX FastCGI Cache) can also cause conflicts; you may need to configure them to work together or temporarily disable them for testing.
  • Investigate WordPress Cron: The preload process uses WP-Cron to schedule subsequent batches of URLs. Use a plugin like WP Crontrol to verify that the wpo_page_cache_preload_continue event is being scheduled correctly. If your host disables WP-Cron, you may need to set up a real server cron job to trigger it.
  • Review Server Error Logs: Check your PHP error logs for any messages that might indicate permission issues or script termination. The cache directory (wp-content/cache/) must be writable by the web server.

2. Scheduled Preload is Unreliable or Drifts Out of Sync

Symptoms: The scheduled preload doesn't run at the expected time. The "Next Run" time for the wpo_page_cache_schedule_preload event drifts later and later, leaving the site uncached for extended periods.

Why it happens: The scheduler calculates the next run time based on when the previous preload finished, not when it started. If the preload itself takes a long time (e.g., 15 minutes for a large site), the next scheduled run will be "cache lifespan + preload duration" later, causing a gradual drift.

Solutions:

  • Adjust the allowed_time_difference constant: For developers, a known workaround is to modify the allowed_time_difference value in the class-wp-optimize-page-cache-preloader.php file. Increasing this value (default is 10 minutes) can prevent the scheduler from incorrectly skipping preload cycles on sites where the preload process itself is slow. Note: Modifying core plugin files is not recommended as changes will be lost on update. This should be considered a temporary fix until the issue is addressed in a plugin update.
  • Monitor Preload Duration: If your preload takes a very long time, consider using the provided filters to throttle it and reduce server load, which may help it finish faster and reduce drift.

3. High Server CPU/Load During Preload

Symptoms: Server CPU usage spikes to 100% during preload, potentially causing timeouts or making the admin dashboard unresponsive.

Why it happens: Preloading simulates visits to every page on your site. On large sites with thousands of URLs (e.g., WooCommerce stores with many products), this creates significant server load.

Solutions:

  • Throttle the preload: Use the following filters in your theme's functions.php file to reduce the intensity of the preload process. Add them one at a time to test their effect. 
    // Increase delay between loading pages to 1 second (default is 500000 microseconds)
add_filter('wpoptimize_preload_delay', function () { return 1000000; });

// Increase the memory threshold before pausing to 20MB (default is 10MB)
add_filter('wpo_page_cache_preload_memory_threshold', function () { return 20971520; });

// Change the pause interval to 300 seconds (default is 600)
add_filter('wpo_page_cache_preload_continue_interval', function () { return 300; });
  • Disable preload on purge: If you update content frequently, the "Preload after purging the cache" option can trigger resource-intensive preloads at unexpected times. Unchecking this option may stabilize your server.

4. Certain Pages or Post Types Are Not Preloaded

Symptoms: The homepage, category pages, or custom post type archives remain uncached even after a preload completes.

Why it happens: The preload function primarily uses your XML sitemap to discover URLs. If a page type is excluded from your sitemap, it likely won't be preloaded. Other conflicts, like certain CDN or optimization plugins (e.g., Cloudflare's official plugin), can also prevent specific pages like the homepage from being cached.

Solutions:

  • Check your sitemap: Ensure that all page types you want to be cached (homepage, categories, tags, custom post types) are included in the sitemap generated by your SEO plugin (e.g., Yoast, Rank Math).
  • Identify plugin conflicts: Temporarily disable other plugins, especially those that interact with CDNs, rewrite URLs, or handle fonts, to see if the issue is resolved. As reported by users, the Cloudflare plugin has been known to interfere with homepage caching.

5. Preload JavaScript and Font Warnings in Browser Console

Symptoms: Browser console shows warnings like "A preload for '...' was not used because the request credentials mode does not match" or warnings about fonts not being used.

Why it happens: The plugin may generate preload tags for resources without the necessary crossorigin attribute, causing browsers to ignore them. Fonts loaded from external sources may require specific handling.

Solutions:

  • This is primarily a cosmetic warning and may not impact actual page load performance. However, for a technical fix, users have successfully used output buffering to add the missing crossorigin="anonymous" attribute to relevant tags. The 'WP-Optimize – Cache, Compress images, Minify & Clean database to boost page speed & performance' team is likely aware of this issue for a future update.

By systematically working through these common issues, you can often resolve problems with WP-Optimize's cache preload feature and ensure your site remains consistently fast for all visitors.

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