iMac extremely slow suddenly Mum's Mid 2007 iMac suddenly became extremely slow a few days ago. It takes over an hour to boot, and once booted it beachballs almost constantly in every app. Clicking an icon on the desktop takes a minute or so to respond.

It didn't gradually get slow, she must turned it on one day and it was like that.

I have replaced the hard drive with one that has no data yet. I booted off a firewire 800 drive with an OS X installation that boots up perfectly and is fast with another mac, but on this mac it is just as slow as the drive I assumed was failing and removed.

Booting from a mountain lion recovery partition is similarly slow (I never actually waited for it to boot, but it was taking way too long at the grey apple logo when I gave up).

What could cause this?
Check the RAM.

Ordinarily bad or improperly seated Ram will prevent booting BUT it is worth checking.

However, this sounds like a hardware problem as you have done so much troubleshooting.

My suggestion is to bite the bullet and take the old Mac into a brick and mortar Apple Store.