If you find that the time you have to hover your mouse cursor over the taskbar icon, for the taskbar thumbnail preview to appear way to much, you can make them appear faster.
Open regedit and navigate to HKEY_CURRENT_USER\Control Panel\Mouse
In the RHS pane, right click on MouseHoverTime. Select Modify. The default value is 400 ms. Set the new value at, say, 100.
Click OK. Exit regedit. Reboot.
You will see that when you hover, the taskbar previews now appear much faster.
Incidentally, MouseHoverTime indicates the amount of time, in milliseconds, that a mouse pointer must remain within an area the size of the MouseHoverSize property in order to generate a mouse hover message.
Whereas, MouseHoverSize indicates the size of the rectangle within which the mouse pointer has to stay for the mouse hover time before a mouse hover message is generated. You can see that all these, including height & width can be changed here.
Do note that MouseHoverTime will customize the hover time globally, as it will adjust the mouse hover delay for everything, including tooltips, taskbar icons, start menu, etc.
I prefer to have previews, tooltips, etc faster, hence have suggested this tip.
If you however wish to only customize the Taskbar Preview Thumbnail Hover Delay, then, INSTEAD, simply add a new DWORD named “ExtendedUIHoverTime” and give it value 100.
UPDATE: Our Taskbar Thumbnail Tweaker for Windows 7 may interest you.