Frank Goossens on "New render-blocking CSS from Autoptimizer"

There are 2 things to consider when disabling AO;
1. you should also empty your quick cache's cache (if not you will have pages in quick cache that still refer to AO-created files, even if AO is deactivated).
2. you return to a situation without AO (so no "inline" or "defer&inline") meaning that the bulk of you un-optimized CSS is render-blocking (as CSS is, by default).

hope this explains,
frank