I'm running LR3 on a 2.6GHz 27" iMac i5, and it certainly isn't low, and I'm handling 28Mb files, no problem.
When you converted your LR2 catalogue did you optimise it? This basically sets up the catalogue with the new fields and get rid of the c**p. It can make a difference. If you haven't optimised the catalogue you could try that and see if it makes any improvements
A lot of the modules in LR3 were rewritten from the ground up, so they may require some extra resources. Having said that I noticed no speed difference comparing LR2 to LR3