[sneaK]

"good specs"
"2.1 ghz"

I don't think anyone would consider that good specs, but when you say with 4 players, does that include bots? Bots are more expensive than normal players. You also have to remember you're running a lower frequency processor with virtualization, so your actual performance is probably even lower than normal. SRCDS is primarily single-threaded, so regardless of additional cores, you won't see an improvement.