The taskbar shows thumbnail previews of opened windows when you hover the mouse over the program icon. By default, it takes a little time for the preview to appear. Make the previews appear faster with this registry tweak:
Use Windows Key +R to get a run box
Enter regedit and hit your ENTER KEY
Navigate to HKEY_CURRENT_USER\Control Panel\Mouse.
On the right, open the MouseHoverTime key and changes its value from the default 400 to around 150. Decreasing it further may cause problems.