Gmod best server setting-up?

I have some problems with my home-pc. I was made server 4-5 months ago and it worked with 30-32 players normally without lags. Now, after few months it works with lags at 10-15 players. How to increase performance of the server?
I heard something about that gmod servers use only 1 CPU. Dont know how true is it.

Here is my “options”:
Windows 7
Intel Core i7-3820 | 3.6 GHZ | 8 CPUs
16GB RAM

I knov, my inglish iz very bread, sry.

8 Cpus? damn

?

You probably mean 8 logical cores.

4 physical + 4 non-physical, i think :\