What is the reason for the long server response?

Question

Server optimization

The site (on wp) is quite well optimized, it loads from a PC in 1.5-2 seconds (the whole), however, for 3-5 requests in a row, the page can load 5-7 seconds (there is no server response for a long time). Compression enabled(9). Php 7.1.2.8 (FastCGI (Nginx + PHP-FPM)). SSL enabled. HSTS too, caching enabled. Virtual server 2 cores, 4 gigabytes (unfortunately without a game video card), 30gb ssd. The server has one domain. What could be the problem?

Answer 1

The problem is solved by clearing the WordPress cache

Answer 2

Few inputs to determine the cause. You need to profile the application, web server, and OS.