How to reduce server response time?

Question

PHP

An annoying problem has arisen. On a server with a fairly decent capacity (i7, 32Gb, 20Tb), decent such friezes began to appear when the server responded.
At the moment TTFB is 4 to 5 seconds. And it doesn't matter whether the result of the script is returned or just a static txt file. I googled about this, but, for example, HostnameLookups is Off, but I did not find other viable solutions. I'm not strong in administration, but suggested that it might be the number of connections. Tried restarting the webserver but no improvement.
Load monitoring did not reveal anything terrible, there is enough RAM and a processor to the head.
What can be wrong?

Answer 1

Possible problems:
1. Channel width/shaper at the host.
2. Network card driver.
3. Loaded storage system (backup / migration in progress or reading problems).

Answer 2

Is it in the server? Maybe Hindu coding is to blame?
Look for bottlenecks
I advise nmon
there you will see the load on the network, disks and cpu