Slow websites – Sites are well optimized

Hi everyone. I have 2 websites on a Bluehost VPS. Both of them worked fine until a week ago. The websites loaded in less than a second. Now, they take 20 seconds to loads. They sites do not crash or show error messages in the front ends, they are just slow. The sites have Wp-Rocket and MaxCDN and the databases are optimized (query monitor shows that the databases work fine).

When the traffic to the sites is low (100 page views per hour), they work fine. When the traffic is “high” (400 page views per hour), the problem starts. A VPS should deal with that traffic easily. I get this error in the Health Site tool:

Error: cURL error 28: Operation timed out after 10001 milliseconds with 0 bytes received (http_request_failed)

I have deactivated all the plugins and themes and then activated them one by one to check if a plugin or theme is causing the issues, but the issues persist with or without plugins and themes. I contacted Bluehost support, they told me the server is not overloading. I checked Cron jobs and they seem to work fine. The websites do not have many Cron jobs. SSL certificates working correctly. I have checked GTMetrix and all the requests work fine and fast; the only problem is the first request: the website trying to conect to the server.

I don’t know else to do. Any piece of advice would be really appreciated.