Reduce TTFB wordpress server response time/response.

I have a site https://www.instafejm.pl and I will have the server <-> site optimized, specifically to reduce the time (response) of the TTFB server while maintaining the current plugins. It will be necessary to analyze what exactly generates such a high load on the server when the site starts. I have a cache plugin, minification, compressed images, I use CDN meaning everything that could be done with plugins is done and plugins are best not to install new ones and not to delete current ones. The site once it loads works ok, jumps between pages smoothly although every 10-20 transitions there is a slowdown for a few seconds until the server embraces the processes and continues smoothly everything works. Now there is no tragedy either, but sometimes the page is slower and on the counter is 10 longer is TTFB than it should be (should be 200ms, max 300ms, and is 1300-2100ms). I don't mean loading the whole page just the time alone until the server responds TTFB.