Selenium lagging when opening more sessions

I am using VPS with 8 CPU cores.

Problem started when I opened around 30 sessions of Chrome web driver. I minimized all browsers to reduce CPU usage and I was tracking tests inside my prompt. Usually 1 test lasts 55 seconds, but when I have more sessions they vary from 1 minute to 3 minutes sometimes. What is cause of the problem here? My CPU usage is around 20 percent of all time.