Multiple servers on one machine cause low FPS?

I have 2 rust servers on one dedic with 4 cores. The first server on 1,2 cores and the second server on 3,4. The first server has ~500 fps, the second ~1000 fps, although both of them have similar entity and player counts. The question - wtf? But when I turn off one of them, fps of the another server increases to ~1000. How does it work?? If the servers are not interconnected, and each of them works on their own cores! Why does one affect the other? I checked this on another dedic, the same result. Help pls.

 

the fps is good better than good you have nothing to worry about