Go to Start -> Run -> REGEDT32 and find/select HKEY_CURRENT_USERControl PanelDesktop. Find the key “MenuShowDelay” and adjust to a setting lower than the default (400 ms). I find that 100 ms is a favorable setting but your mileage may vary. One final note: the effect is very subtle, so you will need to pay close attention to the difference in speed.

NOTICE: Always be careful when editing the registry. You should always make a backup of the registry before making changes. You can make a backup using your registry editor.