Why may be a long response from the server?

Today I noticed that websites started to give the page longer than usual, once thought to be the case in a long SQL or something like that went to watch, but the queries were normal, measured the time of generation of page, using microtime there was no more than 0.01 but loading certain pages is 12 seconds for reliability and even rebooted the server but the problem has not disappeared, and it is not constant but comes in waves, about 3 minutes everything freezes for 2 minutes normal flight and so on, in what direction to dig? Where can you catch the reason?

Example page load from chroma
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I noticed that this problem occurs at the time when the number of httpd processes becomes equal to 202 where to go next until you come up with
September 19th 19 at 00:21
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