Is there a cap on requests on the free plan?

I have an API which is currently just http (not https) in case this might make any difference. It’s quite a busy one with about 5200 requests a min and a response time of about 120ms. I placed it behind Cloudflare and everything was running normally for a couple days then suddenly the response time just kept creeping up until it was like 15+ sec which was causing me issues so had to disable Cloudflare. It went back to 120ms almost instantly so Cloudflare was 100% causing the huge response delay.

Have I hit some sort of cap? If I join the paid plan will this be removed? Or was something else going on?

I have another normal site (not api) also behind Cloudflare (https) which runs perfectly no issues. But has far far less request traffic.

Any ideas would be much appreciated.