How to find out what loads wordpress and increase the site loading speed?

Question

Highload

In general, there is a project, it is on wordpress, it was loaded in 18 seconds.
After some caching (apache mod_expires + Wp super cache [plugin] ), the project started loading in 9-10 seconds.
In such projects, as a rule, there are a lot of plugins. Checking with P3 (also a plugin) showed that they are all loaded for a maximum of 5 seconds, disabling the longest one, the situation has not changed.
There are no big pictures.
But I will still upload them to some kind of CDN, but I think this will not save the situation as a whole.
What else can be done? Will cloudflare help in this case?
How can I find out the reason for the long loading?
The fact is that even the dashboard takes a long time to load, i.e. wordpress itself, not just project pages. It seems to me that I am optimizing the page "in vain".
It is most likely that the plugins are slowing down in this case ?
You can’t disable plugins, they are needed, move to another hosting too (there, by the way, shared hosting, so you can’t configure something serious in the configs, you can’t do serious caching either, because I can’t rewrite the project either.) I did
the project NOT I, but it must somehow be accelerated as much as possible in this limited situation.
I will be glad to any advice.

Answer 1

So, for starters, it’s worth understanding that caching does not eliminate the problem, but only masks it. Well, if even with caching enabled, the download takes more than 1-2 seconds, then there’s some kind of catastrophe inside, it shouldn’t be like that at all. Roughly speaking, somewhere there are some areas where there should be a big red board with a flashing word SYSTEM ERROR. Just something really wild.
Second, saying "some plugin slows down" is incorrect. The code always slows down. A specific section of a specific code. And it doesn’t matter what plugin it is in, in a theme or somewhere else. It is necessary to find exactly the narrow section and correct it.
What to do? First, deploy a local copy. This will at least make it clear how hosting affects the situation. If locally everything is much better - change hosting 100%. If not, turn on debugging, analyze using Xdebug and profiling. We identify bottlenecks (and there are definitely enough of them) and slowly eliminate them.
If this is a small freelance upwork project with a fixed-price of $50, it’s better not to waste time, give it to the client, return the money and close the contract. If there is a budget for this fuss - then dig as I wrote above.

Answer 2

Yes, as an option, one of the plugins slows down. Turn them off one by one and find the culprit. Find an alternative to the culprit.

Answer 3

The communication channel, the return by the web server, the generation of the web page (server code execution time + request time) can slow down. Everything must be measured.