USB mouse, like any other USB device, shares it's bandwidth with other USB devices. And if you have many USB devices working at the same time, you can get higher latency (longer response time) with your USB mouse. Though, there are different polling rates for gaming USB mice, ranging from 125 Hz to 1000 Hz that minimize the latency (e.g 1000 Hz = 1ms response time) but if the system that deals with USB packets hangs, you'll get higher latency either way.
PS/2 mouse, in the other hand, sends dedicated signals to the PC which interrupts anything CPU is currently doing and forces CPU to deal with the PS/2 input. In that sense, PS/2 mouse and KB are far better since you won't get any latency with them.
Another bonus for PS/2 mouse is that...