Do you have TweakUI? I know that setting can be controlled using that
particular PowerToy.
Anyway, fire up REGEDIT and find the following value:
HKEY_CURRENT_USER\Control Panel\Desktop
The String Value called MenuShowDelay is located under that 'folder'. It
controls how long you have to point at a menu (in milliseconds) before the
sub-menu appears. The default is 400. Set it accordingly and reboot.
Let me know if that doesn't fix it.
-skeene