Hmm, we've been experiencing vastly reduced throughput as well (something like 100 kB/s on images >100 kB in size, cache hit). We're on the free plan, though. Makes me wonder if there's actually some throttling going on behind the scenes.
I can only comment on our exact situation, where the response times were as I mentioned them, exactly. I've not experienced that, but I guess it's worth shooting a question to support (though, expect several days on the free plan)
Might this have something to do with people using Cloudflare to share child pornography a couple of years ago? They've been talking about it again recently.