How to optimize php+sql settings loading (long page loading)?

Question

CMS

Hello. Tell me how to be.
Data
Store on
ImageCMS 500k product base
Dedicated server
Problem
Page (product) loading speed in 3 seconds
History
In general, ImageCMS itself is sadly poorly optimized. Heavy queries with millions of rows are generated.
Somehow managed to reduce some of the requests. Enabled caching for displaying categories (when the request went to the product page, most of the load was generated precisely because of the categories).
We connected the memcache (I guess that the CMS itself does not work well with this module).
Moved to a dedicated server, not cheap, perhaps we are hosted by one of the top hosts.
The problem could not be solved. We went to the speed of 2.5-3 seconds.
Actually a question.
According to statistics, it can be seen that the CMS loads the server by 30%. That is, it does not use all the resources.
The programmer determined that the requests are stuck in PHP. That is, the muscle gives everything quickly, without slow requests.
Perhaps there are some Apache, PHP settings that will allow you to use resources to the fullest. Or more extra. enable cache, or buffer settings and so on. It is to solve the problem on the server side, because there is a lot of trouble with the software part (as they explained to me).
Help me please.
If anything I can thank $.
Happy Holidays everyone.

Answer 1

It’s enough to go through the provider so as not to guess what the problem is, the settings will not help here, since it checks on one user, write to the mail if you don’t get it right

Answer 2

visit the browser on the page and see the schedule for downloading scripts.
html/css/js - compress into one, images too, remember about 5 connections per domain for simultaneous content loading. think about moving websockets, Ajax and Angular for client-side page generation

Answer 3

Try php7 or hhvm (test server first!!11)
Is Opcache enabled?
Is Xdebug disabled (it can slow down the server a lot if it is not used)

Answer 4

Ask hosters for a paid audit and configuration. And don't look for strangers. Don't press money.