My PC (Win 7, 64 bit) hasn't had any issues with IWD2 until I entered the second level of the Goblin Warrens. Suddenly the game is running extremely slowly, and eating up a surprising amount of processing power. Anyone have any idea what's going on?

Try this

http://www.sorcerers.net/Games/dl.php?s=IWD2&f=IWD2/IWD2_Slowdown_Fix.zip#

I had a very similar problem (Win 7, 64 bit) that wasn't confined to the Goblin Warrens, and Aqrit's DDrawFix sorted it out. Unlike GOG's Baldur's Gate installers, Icewind Dale 1 & 2 doesn't install it by default. You can get it here.

Thanks guys- do you know if either of those patches will mess with saved games?

I don't know anything about the first one, but DDrawFix does not affect saved games in any way.

WOOO! Thank you! That's been bugging me for a few months now. I could get the whole rest of the game running silky smooth except those blasted warrens. Works like a charm with Wineskin on a Mac 10.7.5. Very much obliged! Seems to have no issue with save games or anything. Cheers!

I have this problem on my realtive beast of a computer, and I was ripping my hair out over it for a while. Finally managed to google the correct stuff, and my google-fu had me end up here.

Olnorton; I love you, I love you, I love you, I love you, I friggin love you for making/finding this fix, and now I can relax and have the epic adventure that IWD2 actually is.
Thank you.