High loading and caching

Good day.

My situation .. The site has only about 15 plug-ins installed (all plug-ins are light, except for the access and payment distribution plug-in), css file ~ 1.5 MB.

The server has 8 GB of RAM and a hard disk of 100 GB, it is in the cloud, the processor has 4 cores, hosting a well-known hoster.

We do load testing via loader.io – with 30 simultaneous entries to the page, its opening time increases from 1.5 seconds to 6 seconds. Errors begin (from 5% to 17%, several times it was 30%). Php is very long processed. The site is unavailable (timeout) already with 40 simultaneous entries of users to the page (now on another platform we have about 350-400 simultaneous entries per second – at the time of the new article / news release). In the near future, there are about 1,000 people at the same time.

Question 1. Why is the theme so heavy on the processor? How can I fix this?

Question 2. If you put a caching plugin, then:

A) users cannot log in (although the system accepted their data) – after entering the username and password, users see the same page for unauthorized (hangs the “Authorize” button instead of “author name”, comments are not visible, and other features for authorized users);

B) after authorization, the user sees a page with hidden content in the same form as unauthorized: that is, it sees not the content, but the text that offers to log in and buy access to the site. A previously authorized user, having paid, sees a page for payment, but not hidden content. How to deal with such problems?

In essence, caching needs to be disabled, otherwise it hides 3,500 pages for users who pay for access. But without it, the site drops already at -40 simultaneous user logins. When caching (php), this topic WordPress keeps the load (1000 users at a time), without caching only 15-17 – clean without errors and with a download speed of about 3 seconds per page (this is ” limit of patience “user).