When 'Upgrade' means 'Downgrade'

I just installed CS5 Design Suite on my Mac Pro a week ago, and tonight, I've just de-installed it. I'm now back running CS4, because with CS4, I get performance. I fail to understand why we 'upgrade'. I think the term 'upgrade' is used rather loosely by the software industry. CS5 is like CS2.... Slooooowwwwww!