Windows 7 x64. ASUS P8H67-I. Intel video.
I've tried five different wireless mice, three from Microsoft (two USB dongle, the other Bluetooth) and two wireless from Logitech (both dongle). I've reinstalled all USB drivers and Bluetooth drivers and have installed the latest Intel 64-bit video drivers.
With a two monitor setup, there is truly horrible mouse-lag, the left monitor worse than the right. It is like trying to point at a single letter or number on a blackboard using a slender tree branch that is 15-foot long and very flexible. You just can't aim it accurately and it has "whip" which causes the tip to wander and overshoot the mark. It isn't just slowness.
A wired mouse does not have the problem.
Putting the mouse very close to the dongle has no beneficial effect. It's not a distance issue. Fresh batteries.
Putting the dongle in a different USB port has no beneficial effect.
Sometimes turning off the mouse for a few seconds and turning it back on will give me a few minutes of acceptable use. But it quickly deteriorates.