A bunch of us over on the MacOSX forum have discussed an issue where the Dell Mini9 slows down dramatically when it's getting warm. Running Geekbench, we were getting performance numbers 2x-5x less than what was expected. It seems that the CPU throttles itself even when it shouldn't.
I returned mine to Dell, and they replaced the motherboard. With Ubuntu, I know consistently get a benchmark number of 1,000. With MacOSX, I get consistent numbers over 900. Before the repairs, I'd get as little as 150-200 sometimes.
I'd definitely recommend running a number of Geekbench runs, and make sure you get consistent results (see expected numbers for Ubuntu and MacOSX above). If you get much less than that (150-500 would be an indicator), I suggest contacting Dell immediately.