Features of SSI in Highload?

Question

Server Side Includes

There are:
1) Self-written site on Yii,
2) SMF 2.0 forum
3) highload.
Task:
1) Connect the Site and the forum to achieve maximum performance
2) The forum should be in the site template.
Implementation option:
1) Divide the site template into 3 SSIs
..... a) Header (unique for the user, 6-10 different parameters): php script execution using yii, saving an array of unique data in memcache for a certain time, the request goes on the backend
..... b) Left Block (Common for users): Save to the nginx file cache, take from there, bypassing the backend
..... c) Footer (Common for users): Save to the nginx file cache, take from there , bypassing the backend
2) Connecting SSI on the forum
3) Implementation of general authorization
Question:
1) It is confusing that in this case there are 2 requests to the backend when viewing forum pages, incl. using yii, are there any big disadvantages in this case, is it worth bothering about this?
2) Are there options for another implementation that will not be very technically complex and provide greater performance.
PS. Intuition says that I complicate.

Answer 1

If your header depends only on the user (uid) and does not change over time - put it in the cache with the uid key and let nginx take it from there, there will be one less request to the backend.
But in your task, the biggest load will be created not by your header, but by SMF, I assume) Set up caching in memcached in it, etc.

Answer 2

Install a more powerful server and forget about caching up to the first million hits per day.