Is the game just damn slow, or not responding at all?
try to run the game in windowed mode (see video options) and play around with the video options. If that doesn't help, try to delete your config.cfg (in your application data folder - see FAQ for more info about it)
if it's not responding at all, it's maybe an endless loop - when exactly do you encounter this slowdown? after loading a savegame? new campaign, too? during tactical missions? on geoscape - please be as verbose as possible.