Performance plugins slowing down

Something must have happened two weeks ago. Any idea what it was?
“speed decreases to 0” – literally? What exactly happens?
Have you tried the usual troubleshooting steps – e.g. switch to a 20-something theme, disable all plugins, run Health Check?
I don’t use Autooptimize but my understanding is Cloudflare can do much of the same stuff – minimize css, javascript, and html for example – and doing it twice can break things.
Have you tried a Cloudflare cache-everything page rule as an alternative – or addition – to WPFC?
For what it’s worth, your site seems quick to me but webpagetest.org disagrees. TTFB seems to be the main culprit – from my limited understanding that’s a usually a hosting issue.

The domain was renewed, but I already asked the host for any changes or issues made by them, they said everything is fine. Then they updated the newest version of PHP and cleared the cache. Still the same…

What I noticed is that the problem is with the cache as a whole even if the plugin is deactivated. When I clear the Cloudflare cache the speed increases, but on the second visit drops to 0. Literally 0.

I found the problem, an extension of AMP plugin – Progressive Web App. Thank you for your help!