Will Mountain Lion make an older computer run faster or slower? Performance/responsiveness is a common concern with new software, especially on older machines. In general, upgrading from Snow Leopard (10.6) to Lion (10.7) wasn't too bad, but now Mountain Lion (10.8) is adding even more features such as Notification Center. 

What will be the performance impact of Mountain Lion on Macs which are nearing the lowest end of the system requirements?
Faster. 

I have a Mid 2007 iMac 2.4 GHz Intel Core 2 Duo with 4GB RAM. This is the oldest iMac model that can officially run Mountain Lion (according to Apple's tech specs). 

I upgraded the first day and haven't noticed any slowdowns so far. Performance actually seems faster in Finder, Mail, Preview, etc. 

Safari stands out the most in terms of noticeable speed improvements.