var lastEvent;
function checkMouseMove (event){
var dx = 0;
var dy = 0;
if (lastEvent) {
dx = event.clientX - lastEvent.clientX;
dy = event.clientY - lastEvent.clientY;
}
var d = Math.sqrt(Math.pow(dx, 2) + Math.pow(dy, 2));
if (d > 5) { /* handle mouse move here */ }
lastEvent = event;
}