Does the plugin consume server resources?

If your Joomla website is very complex and has a lot of CSS and Javascript files to analyze, parse and combine, the plugin could require an important amount of resources to the server to accomplish this operation.
All resources are processed at once and cached for a later usage, in order to avoid to repeat the same operation continuosly. If you experience a high CPU usage or disk space needed to store cached files, you can exclude certain files or extensions from the optimization process.

If the problem is limited to the CPU usage only, you can try increasing the cache lifetime so that the optimization process will run fewer times: