My nephew upgraded his very old laptop, from 2009 I believe, to Yosemite and hates it because it runs so slow. He only has whatever original memory came with it and I'm sure that may be part of the problem.

Anyway, he wants to go back to 10.6. I told him he will have to move all his files to an external HD, wipe his HD, clean install 10.6 and then move his files back.

Is that about right? Thanks in advance. I've never got backwards on my Mac Mini's so not sure if that's even possible.

Regarding speed, one thing that can make a significant difference...

Go to System Preferences -> Accessibility -> Display and check Reduce transparency.

This reduces CPU usage and improves graphic performance for me.

You haven't given us enough information to know what laptop your nephew has, but it is entirely possible that it is capable of additional memory which would probably proved better speed but the processors used in 2009 even late 2009 were not all that fast to begin with. However, if I may offer an observation having gone through several Yosemite beta versions and now the release version. It appears that after installation Yosemite continues to do some optimizing and the performance continues to improve for some period of time. It might be worth your nephew waiting a few days to see if things improve.

Joe, I'm glad you mentioned that Yosemite continues to optimize after the install. I noticed that when got my new, 2012, Mini a few months ago that things improved slowly over a week or so.

The nephew is out of town for a new days so won't be able to talk to him until later this week.

I agree that he could upgrade memory but he's on a pretty tight budget and may not be able to spring for it but I'll talk to him when he gets back.