I suspect this problem is two-fold.
First off, consider that Access 2000 vs Access 2007 probably have different system requirements. If these are older computers that ran A2000 just fine, I would expect a performance hit when upgrading to a modern software release.
Second, I've noticed that upgraded databases run slower than databases that are designed from the start in A2007. ALSO, I just found this hotfix posting today on the Access Team blog, which is intended for A2007 SP1. (Released August 26, 2008). Among the fixes, it supposedly addresses this 'upgrade slowdown'.
Access 2007 SP1 hotfix package Access Team Blog: A2003 and A2007 hot fixes
I would recommend upgrading all your Access & Runtime installations to SP1, and then if this doesnt help the problem, try testing the hotfix on a single pc first.