Thanks Marc. The site started to go down some days ago. To improve, a more powerful VPS was taken, without obtaining results. So a series of optimizations were done, mainly on expires and caching, which took it from Grade E to Grade B. This improved things a bit but still takes 30 seconds to send a post with two lines of text and 20 seconds to switch to a new one page. The provider says everything is fine on his side.
However, the problem remains also "This comment can no longer be edited", I tried to disable mod_security without getting results.