Unlike the SOURCE games which are fully optimized for PCs, ME was a console port and it was done so by a small team.
You've probably already tried this, but turning off VSync -- if it's on -- will double to triple the frame-rate in some cases.
Also makes sure audio is set to hardware, if the MacBook supports it, since this would free up the CPU a little bit.
Another option is to run at a lower resolution and use a 4:3 aspect, if turning all settings to low doesn't help.
I played ME under XP64. I know there's a little drop in frame rate compare to XP 32, but I also know that the extra memory helps with these newer games, especially SOURCE games. I'll get an tiny hiccup when a new area loads in, but then all is smooth.
Mass Effect is an excellent game, so even if you're unable to get it running smoothly all of the time, it's worth playing. The areas you mentioned are the largest in the game, but most of the game takes place in small areas.
I'm on my second play through and I've already encountered some new aspects of the game that I had missed -- I'm thinking they were times events. Overall it's one of my favorite games, so I hope you'll find a happy medium and not give up on it. There's always a dedicated PC -- it will cost under a grand and run this game like butter...
