What affects the loading speed of the site after idle time?

Question

1C-Bitrix

Bitrix site php 5.6, mysql 5.6, 8 cores, 8 RAM, user - just me (private access). Spinning on a virtual machine on a server, as I remember - VMware.
If you leave the tab open for at least 10 minutes (or you can close the tab - the effect is the same), the page will load in 30-50 seconds.
If you leave the site alone for a shorter time, for example 1-2 minutes, then the page will load in 4-5 seconds.
If you reload the page in a row, its loading speed will fluctuate between 0.9 and 1.2 seconds.
What might cause such behavior?
In the Network tab in the browser, the following picture - only the waiting time changes (Waiting [TTFB]).