Obviously your resources are lower in the virtual machine, so performance loss is indeed to be expected. I guess it would vary from game to game. I tried Age of Empires II a couple weeks ago on a virtual Windows XP (about 1 Go of RAM, one processor, 90 Mo video memory) and I could run the first tutorials without any problem. But when I got to the tutorial mission with a bigger map and a second computer, the game got excessively slow, so it would be unplayable for most scenarios. Given how old the game is, I wouldn't expect much for other games, but my settings are probably not optimal and of course you'll have better chances with better hardware.
That's just my personal experience, I guess others can give you better insight.
Disclaimer: I'm a noob, not a dev; my advice is only a guess.
Attention : Je suis aussi noob que vous, prenez mes suggestions avec un grain de sel ;)