I can NOT shed light on Why, in fact I am looking for the answer on that myself here:
http://annoyances-resolved.********....l-3000-by.html
At that URL you'll find the solution - just put Vista to sleep. Actually, it is not a real solution, just a fastest workaround so far.
IT still takes a while because, well, the mouse is sluggish, duh!
So let me know if you find a better solution, you can post it here, there or both!
Thanks in advance