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Troubleshooting WP-Optimize Image Compression Issues: A Comprehensive Guide

20 threads Sep 7, 2025

Content

Many users of the 'WP-Optimize – Cache, Compress images, Minify & Clean database to boost page speed & performance' plugin encounter issues with its image compression feature. This guide compiles the most common problems and their solutions, based on community reports and troubleshooting.

Common Image Compression Issues

Users frequently report the following problems:

  • Bulk compression processes only one image per run.
  • Automatic compression of new images does not trigger.
  • Errors related to a semaphore lock, such as Lock (updraft_lock_smush) could not be acquired.
  • WebP conversion not working during bulk operations.
  • High server resource usage during compression.
  • Conflicts with other image optimization plugins.

Why These Issues Happen

These problems typically stem from a few key areas:

  1. Stuck Processes: The most common cause for bulk compression failures is a stuck process lock. The plugin uses a semaphore lock (updraft_lock_smush) to prevent multiple compression tasks from running simultaneously. If a process crashes, this lock may not be released, blocking all subsequent tasks.
  2. Plugin Conflicts: Having remnants of other image optimization plugins (like Smush or EWWW Image Optimizer) or running them concurrently can cause conflicts. These plugins often leave database entries that WP-Optimize detects, leading to confusion over which images are already processed.
  3. Server Configuration: Timeouts, limited memory, or restrictive PHP settings can interrupt the connection to the reSmush.it API or halt the compression process prematurely.
  4. Multisite Incompatibility: On WordPress Multisite installations, a known code issue can prevent the compression of all image sizes during a bulk operation.
  5. Database Load: The image compression feature creates several metadata entries for each image. On sites with very large media libraries, queries related to this metadata can become slow and resource-intensive.

Recommended Solutions

If you are experiencing issues with WP-Optimize's image compression, try these steps in order.

1. Clear the Stuck Process Lock

The error Lock (updraft_lock_smush) could not be acquired indicates a stuck process. The lock should automatically expire after one minute. If it persists, you can often resolve it by waiting a few minutes and trying the compression again. In severe cases, a manual fix may be required. The 'WP-Optimize – Cache, Compress images, Minify & Clean database to boost page speed & performance' team suggests that the lock is designed to clear itself, but patience is the first step.

2. Check for and Remove Plugin Conflicts

Ensure no other image optimization plugins are active. If you have previously used another plugin like Smush, you may need to clean its leftover metadata from your wp_postmeta table. Be very careful when manually deleting database entries; always create a full backup first. Meta keys from other plugins, such as wp-smpro-smush-data, are not used by WP-Optimize and can be safely removed if you are no longer using those plugins.

3. Review the Logs

Always check the plugin's log viewer after a failed compression attempt. The logs often contain specific error codes (like max_attempts_exceeded or cURL error 28) that point to the root cause, such as an API timeout or a server connectivity issue.

4. Verify Server Settings

Compressing thousands of images requires adequate server resources. If you see timeouts or resource limit errors, consult your hosting provider about increasing your PHP's max_execution_time and memory_limit. For API connection timeouts, the issue may be with the reSmush.it service, and trying again later is advised.

5. Multisite Fix

For Multisite users reporting that bulk compression only affects the full-size image, this is a recognized code incompatibility. A code modification, as mentioned in the community threads, may be necessary to ensure the plugin switches to the correct blog context during compression.

6. Manage Database Impact

Be aware that enabling image compression will add several metadata entries per image. For large sites, this will increase the size of your postmeta table. Regularly using the plugin's database cleanup tools can help manage this growth.

Conclusion

Most WP-Optimize image compression issues are temporary and resolvable. The most effective strategy is to eliminate conflicts, ensure robust server settings, and use the built-in logs for diagnosis. For persistent problems, the community forums are a valuable resource for finding solutions that have worked for other users.

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