Used CSS generation timed out occurs when WP Rocket fails to retrieve critical CSS from its service. The request stops because the server cannot reach the WP Rocket endpoint or the process exceeds the allowed time. Common triggers include blocked outbound connections, security plugin blocks, cron failures, server overload, license issues, or local environment settings.
When the request stops the plugin does not inject critical CSS. Pages load without inline styles for above‑the‑fold content. Metrics show missing critical CSS and render‑blocking resources.