In Windows 7, if you hover the mouse over any running application’s button in the Taskbar, it shows the thumbnail preview of the running application with a close button. And if you hover your mouse cursor over the small thumbnail, Windows shows a live preview of the running application and it makes all other running applications windows transparent. If you are looking for a way to adjust this live preview delay time then this post will help.
First of all launch the Registry Editor by clicking Start, then type regedit and hit Enter.
Now in the left sidebar, navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced and you will be able to see the ThumbnailLivePreviewHoverTime parameter in the right side main window.
Double click it and set the new value in milliseconds, for a faster preview you can set the delay time up to 1000 milliseconds.
Click OK and you are done.