When mouse is hovered on the program icon in Windows 7, you may notice that it does have some latency to display the preview after mouse over the icon. To speed up the thumbnails preview, you can change the delay time via Registry.
To change the delay time by following the steps below:
1. Click Start then in Search Box Type regedit.
2. Navigate to HKEY_CURRENT_USER\Control Panel\Mouse.
3. On the right panel, look for MouseHoverTime and double click on it. Change the value to 0 (in millisecond) then click OK button to save the changes.

4. Close the registry editor and restart your computer in order to take the effect of the changes.
After Restart, you can verify the delay time taken to display the thumbnail preview is faster than the previous.
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