It's the ying-yang balance. You sped up the rest of the machine so to keep the machine in balance, the mouse has to go slowly. If it didn't then it would end up getting faster and faster, eventually breaking the speed of light and traveling in time. This of course would have the unfortunate side-effect of having it disappear from your desk.

In short, it's a defensive measure coded into the OS by Apple to stop their machines from disappearing into swirly-whirly time-hole things. M$ tried something similar with Vista but got the balance wrong