Your mouse's polling rate determines how often it sends data to your operating system. All mouse movements (counts) and button presses must wait in the mouse until windows polls for them. By default, windows will poll a usb mouse every 8 milliseconds (125hz).
You can increase the frequency of polling with a number of programs/drivers/registry fixes. All this does is reduce the delay of sending the mouse data to the operating system. This has no effect on your sensitivity, but it will increase cpu usage.
For example, say you are using a polling rate of 500hz (2ms). Over a period of 8ms, you move your mouse 40 pixels (constant movement, no acceleration). Every 2ms your mouse sends a count of 10 pixels. Now you increase your polling rate to 1000hz (1ms) and make the same movement. Instead of 10 pixels every 2ms, your mouse now sends 5 every 1ms. Either way, 10x4=40, and 5x8=40. same distance, one is just polled more frequently.
check your polling rate:
mouse movement recorder
direct input mouse rate
change your polling rate:
hidusbf - windows xp
hidusbf - windows vista/7 (requires slightly more effort)
If you decide to increase your polling rate, make sure your mouse can consistently provide updates at the increased frequency. If it cannot, the delay of your input will change.
Your mouse may not be able to report consistently for any number of reasons, including your operating system, cpu, motherboard, or the mouse itself.
For example, I have two screenshots taken of mouse movement recorder with two different mice overclocked to 1000hz. Both screenshots are the end of a short movement of each mouse. The first screenshot is of the wheel mouse optical 1.1a. You can see that every time windows polls (1ms), the mouse has data to send, so the rate sticks to 1000hz (or very close to it). As my movement begins to slow down, and eventually stop, the mouse rate goes down. This is because windows is polling every 1ms, but as the mouse moves slower, sometimes there is no movement at that poll. Then another 1ms later, windows polls the mouse and there is movement, so the period of that movement was 2ms long instead of 1. Frequency = 1/time, so 1/(2ms) = 500Hz. Because windows polls every 1ms, the periods can only be 1ms, or 2ms, or 3ms, etc, and then the hz can only be 1000/1, 1000/2, 1000/3, 1000/4, 1000/5, etc (1000, 500, 333, 250, 200). This explains why the mouse eventually hits 333/200hz before the movement comes to an end.
In the second screenshot of the mx518 @ 1000hz, we don't see as good of a performance. More than 5 times the mouse reports at more than 1ms before it is slowed down. Using mouserate checker, I put together a screenshot comparing the 518 at 500 and 1000hz. You can see at constant movement that the 518 reports correctly every 2ms, but sometimes even lower when set to report every 1ms. Looking again at mouse movement recorder at 500hz this time, you can see the report rate looks as expected as the mouse movement comes to an end.
If you overclock your mouse and it doesn't behave as expected, try stepping it down to the next polling rate to find something stable.