Ive noticed a bit of traffic flowing into this blog concerning the problem of HG:L slowing down and locking up.
To be quite honest i suffer the same problem and dont have a solution for it im afraid!
I can personally play the game for some time and will run superb, and then for no given reason it will suddenly just slow to an absolute crawl... in game im talking 1-3FPS at anyone time and trying to transfer from 'level to level' on the loading screen can take forever sometimes!
This is normally sorted out by a restart of the game, but this is annoying and shouldnt have to slow you down in such a way!
Ive Google'd the problem myself and the most viable comments on the matter i have come across are to keep you disc defragmented properly and RAM can be a big player with it all (as per usual, i suppose) the HG:L .exe files also seem to be optimised for Large Address Set too, so it shouldn't struggle if plenty of RAM is available.
Personally i think it may be more of an 'early days' teething problem and will be ironed out as new updates are released, in which reading the various internet tips, one will be released sometime this month (December '07)