So I have an older MacMini. It's not the world's greatest machine, but it doesn't suck that badly:
Intel® Core™2 CPU T5600 @ 1.83GHz × 2, 1gig ram
I have installed Ubuntu on MUCH weaker machines with acceptable performance. I have installed Windows 7 on this machine with acceptable performance. But Ubuntu 12.04? SLOW. Horribly slow. Loading the system dialog is a 45 second affair. Starting chrome is a 2 minute affair. Launching Ubuntu Software Center? 5+ minutes. Something obviously isn't doing what it's supposed to be.
Any ideas where to start looking?