I've tried to throw hardware at it but what I've found is that sorting is the biggest killer of performance on large Inboxes. Once that sort option is associated with your user preferences (users table, you'll see "message_sort_col"), things will slow down. After all the other stuff I tried, I "null"ed mine out as a test and my Gmail "All Mail" folder can pop up in around 4 secs (60k messages).