I had the same problem, and after much internet trawling and scratching of my head, I fixed it. I remembered how I got Mass Effect to run on my PC, and it was by disabling my DVD software. I use power DVD, but it interfered with the game. So i ctrl+alt+del it and closed Pdvd down manually (Also closed down the itunes and ipod programs too, so might have been them instead, although i doubt it). Now there's no more menu/inventory lag and the game is 100x better for it!
Hope this works for everyone else too!
Quintus