Grand Theft Auto IV for the PC, in its initial release, would slow down pretty horribly if there was more than like four cars on the road. Even with a reasonably powerful computer, it was pretty bad to play (though updates did increase performance considerably). I remember when I first played it thinking "you know, Serious Sam came out like seven years ago on crappier hardware and it could handle way more stuff while maintaining a fast frame rate"

It's obviously not apples to apples, the models in GTA IV are much more detailed, but it was still pretty disappointing to play initially.

It's still my favorite GTA game, but I ended up playing through the 360 version my first time around.