You shouldn't have to live with this; it sounds like Lightroom is fighting with other processes on your system. Clearly, the catalog size is not the issue; some other ideas:
1. Make sure the LR pref for Auto Saving XMP is OFF. Auto Save XMP is a disaster waiting to happen; save out your metadata manually and frequently.
2. Take a look at whether LR performance varies with other apps running. Especially stuff running in the background, like anti-virus etc.
3. Leave plenty of free disk space on all drives LR knows about, including system disk (at least 20% at all times).
4. Clear the ACR cache through Bridge.
5. Check hardware (like RAM) for performance/failure condition and verify all settings for drivers and hardware controllers. For example, Open GL settings on various video cards have been reported to cause problems (but not on Mac).
...With the above in mind, it could be that Lightroom is having trouble generating previews. That can always bring performance to a crawl. Check all the preview status and prefs; delete previews and see if that makes a difference.
Since you're having the same problem with both machines it seems to me like it's a software config that's similar between the two, and not hardware. And not LR. Could be a set of files (a folder or collection) that is wonky and can't resolve the necessary operations.
Although it would appear that LR is misbehaving, I think it's much more likely that there is some other system-level process that is interfering.
Try both 64-bit and 32-bit. I don't expect you will see a difference.
I always pay close attention to the performance problems that others report; I have had none. OS X... XP... Vista, shouldn't matter. Properly configured, LR 2.1 can fly on all kinds of systems.
Find a way to determine the cause of the problem and I believe you should expect to be completely satisfied with LR's performance. If it's not a LR pref; it's a system or software setting elsewhere.
...One final thought - for years, working in prepress, I was plagued by what might seem the most benign of troubles - corrupt system fonts.
If you're having inexplicable trouble be prepared to examine absolutely everything.
A while back I made a post on my blog
about LR performance; might be worth a fresh look.