Hi uomopalese;
Indeed; cfr. this quote from the plugin's homepage:
If you consider performance important, you really should use a caching-plugin such as e.g. WP Super Cache or HyperCache to complement Autoptimize.
Autoptimize optimezes CSS & JS, but that slows things somewhat down if used without a page caching plugin (because in that case AO gets triggered on each request, which is really not needed). So do install a page caching plugin and your site will fly.
hope this helps,
frank