The default speed of the Start Menu is pretty slow, but you can fix that by editing a Registry Key.
Fire up the Registry Editor and navigate to the following key:
HKEY_CURRENT_USER \ Control Panel \ Desktop \ MenuShowDelay
By default, the value is 400.
Change this to a smaller value, such as 0, to speed it up.
If this doesn't work for some reason, then you might try the following:
Navigate to Display Properties then Appearance then Effects and turn off the option titled Show menu shadow. You will get much better overall performance.