Thank you for your post.
Can you tell me more about the machines? Can you be a little more specific about the slowness? That is, what is the difference in speed? How many files are being accessed by the clients? Is this Mac OS X 10.6 or Mac OS X 10.6.1? Are any scripts running?
I am running FileMaker Pro 10.0v3 on both Mac OS X 10.5.8 and Mac OS X 10.6.1, and I do not notice any difference in speed, so anything I can do to replicate your slowness issue would be appreciated.