This little tweak speeds up the display of the taskbar thumbnails.
1. Start regedit.
2. Navigate to HKEY_CURRENT_USERControl PanelMouse.
3. On the right panel, look for MouseHoverTime and double click on it. Change the value from 400 to 0 (in millisecond) then click OK button to save the changes. Reboot or logoff/login and enjoy instant thumbnail previews.