Quote from IA's F.A.Q about slowdown problems:
QUOTE
1- Make sure that you have followed the mod's installation instructions accurately. In particular, make sure that you have NOT installed BG2 TweakPack.
2- Make sure that you don't have a bottomless bag of holding. (Also, make sure that you have not installed any other mod which adds new containers (bags, etc) to the game.)
3- Make sure that the 3d acceleration option is not turned on in your game's configuration.
4- Make sure that you have made a clean installation of the game before installing your current mods. Leftovers of old mods (even after uninstalling them) may cause such problems.
5- Make sure that a "Real Time Virus Protection" is not running in the background on your machine.
6- Make sure that your machine (if an old one) is not continuously running for more than 12 hours without a restart. A restart will help the machine to refresh its resources.
7- Make sure that the hard disk has at least 15% free space (both the directory in which windows is installed and the directory in which BG2 is installed).
8- Defragment your computer.
Other than that I have no idea what may cause the lags. We have tested IA even on slow and old computers and didn't have serious lagging problems at all. If the computer is slow and old, it is natural to experience some slowdowns in some of the heavy battles, but not when your party are just walking normally.
Edited for typos