Just so you know this game isn't a port,
They had 3 teams, 1 xbox 360, 1 ps3 and 1 PC where as GTA IV was a port, hence the bad performance.
GTA iv looks miles better than this though and this doesn't run too well at all either.
The only difference is that gta iv actually needs a good cpu next to a good gpu, while this only needs a good gpu, this annoyed the people with dual core cpu's.
The bad performance is simply because GTA IV always already had bad performance on the consoles, that was at 1280x720, now people play it at 1920x1200, a resolution
TWO AND A HALF TIMES as big as the crap on the consoles, it's only logical to assume it'll run a lot worse. Also, they turn the traffic, draw distance, detail distance, shadows, filtering, etcetc
FAR higher than the console crap, again impacting performance. Sure pc's hardware is a lot faster, not THAT fast though.
GTA IV is not such a bad port as people seem to make of it, turn your res down and the details to the console crap, and it'll fly.
At 1280x1024, 15 traffic, 40 detail distance, 20 draw distance, GTA IV ran at 45 average fps ( 58 fps in the benchy), never dropping below 30, that is a nearly 50% higher framerate than you get on the playstation and xbox... That was with a quad at 2.8 ghz and a geforce 8800GTS 512. Not the fastest pc spec by any means.