Work units that are submitted with a CPU time of only a few seconds or minutes is because of three reasons (in order of probability)
1. There is too much radio interference in the WU, so the client "gives up" because it will never detect anything
2. There is a hardwarre problem with your computer, such as dodgy RAM (i've had this on one of the assimilations i made at college)
3. You are using a hacked client
The first one is the most common, and will give you one result like this, and normally not very many of them. The second option will give you lots of short WUs from that computer, and it needs attention. The third, you are a total %$*"%£$"!^"$ who only wants to get to the top, even if it means sacrificing the integrity and point behind the entire project.
And yes, Lithium381, the CPU speed is normally the only factor that affects the WU time, as well as the architecture of the CPU. Athlon XPs rock at Seti, due to their good FPU (Floating Point Unit), whereas K6-x processors suck at Seti because they have weak FPUs.
ConfusedBW