Please help me understand the server load

Good afternoon, dear colleagues.
There was such a need, you need to analyze (conduct an independent audit) of the web server.

The fact is that the server hosts a fairly large number of sites, but one of them, the main site, is very slow. Normal LAMP.
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But you need to understand in what (narrow) place it all starts to stop. Because surprisingly, the rest of the sites hosted on this server seem to work. And the main one is sometimes loaded, sometimes it is not loaded at all.

Please help me, how can I determine the server load, i.e. what scripts, or what processes most of all load the server.

UPD: htop, top, helps of course, but he writes the total load of the application. Those. somehow, the postgres server eats up 60% of the CPU. (ping-pong, i.e. not always). That hangs in the top, it does not hang. Once a minute. There are no huge queries to the database.