Why WP Super Cache Might Not Reduce Your TTFB (And How to Fix It)

If you've installed WP Super Cache expecting a dramatic improvement in your site's Time To First Byte (TTFB) but are still seeing slow results on tools like GTmetrix or Pingdom, you're not alone. This is a common point of confusion for many users. This guide will explain why this happens and walk you through the most effective troubleshooting steps.

Understanding the Core Issue

WP Super Cache works by generating static HTML files of your dynamic WordPress pages. When a visitor arrives, the server can deliver this pre-built file almost instantly, drastically reducing the TTFB. However, several factors can prevent this process from working correctly, leading to the server falling back to generating the page slowly from scratch (a "cache miss"). The symptoms often include:

• Great TTFB on the homepage but slow TTFB on other pages.
• Caching appears to work initially but stops after a few hours or days.
• Performance tests show no improvement or even regression.
• Test results are inconsistent between different tools (e.g., GTmetrix vs. Pingdom).

Most Common Causes and Solutions

1. The Debug Log is Your Best Friend

The single most important step is to enable the plugin's debug logging feature. This log will tell you definitively if a page is being served from the cache (a "cache hit") or not (a "cache miss"). It will also provide clues as to why a page isn't being cached.

1. Go to WP Super Cache -> Settings -> Debug.
2. Check Enable Debugging and Enable Logging.
3. Click Update Status.
4. Now, visit your site and run a speed test. Check the debug log (/wp-content/cache/supercachedebug.txt) to see the results.

Look for entries that say "supercache dir: /path/to/cache/page.html" which indicates a successful cache hit. If you see "No wp-cache file exists," then the page is not being served from the cache.

2. SSL/HTTPS Configuration Problems

Installing an SSL certificate can sometimes break the caching mechanism if the plugin's configuration isn't updated. If caching worked perfectly before SSL but stopped afterward, this is a likely culprit.

• Go to WP Super Cache -> Settings -> Advanced.
• Find the Miscellaneous section.
• Ensure that HTTPS detection is enabled. The WP Super Cache team suggests using the "HTTP_X_FORWARDED_PROTO" method, but you may need to try different options if issues persist.
• After changing this setting, always clear the entire cache.

3. Cache Not Serving for All Visitors or Tools

Some performance testing tools, particularly Pingdom, can sometimes send headers that prevent them from receiving a cached version of the page. This is why you might see a fast, cached TTFB on GTmetrix but a slow, uncached TTFB on Pingdom for the same URL. This is often a configuration issue on the server or with the tool itself, not a fault of the plugin. The debug log will confirm if the page is generally being cached for other visitors.

4. Cache Being Automatically Cleared or Corrupted

If the cache works for a few hours and then disables itself, something is likely regularly clearing the cache folder. This could be:

• Another Plugin: A security, optimization, or management plugin might be aggressively clearing caches.
• Server Cron Jobs: Some web hosts run cleanup scripts that delete files from the cache directory.
• Disk Space: If your server runs out of disk space, it can prevent new cache files from being written.

Check your other plugins' settings for any automatic cache clearance options. If the problem is severe and recurring, a workaround mentioned by some users is to completely delete and reinstall WP Super Cache, which clears out any potentially corrupted data.

5. Misunderstanding "Leverage Browser Caching"

A very common point of confusion is that WP Super Cache will fix a "Leverage Browser Caching" warning on GTmetrix. This is incorrect. This warning refers to how a visitor's browser caches static files (like images, CSS, and JS), not how your server caches pages. WP Super Cache is a server-side caching solution. To fix browser caching warnings, you need to configure your server or a CDN like Cloudflare to send appropriate expiration headers for static resources.

Conclusion

When WP Super Cache isn't improving TTFB, the problem is almost always a configuration or environmental issue preventing the cached files from being created or served. Start with the debug log—it will provide the evidence you need to diagnose the specific problem, whether it's related to SSL, a conflicting plugin, or server settings. By methodically working through these common causes, you can get your server-side caching working efficiently and enjoy those dramatically reduced load times.