In past we have covered how to guide on speeding up taskbar thumbnails in Windows 7 also how to disable taskbar thumbnail preview. There are tons of tweaks if you know which registry key value to be changed in Windows 7 registry. For e.g you can resize taskbar thumbnails size by editing HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Taskband registry key.
If you are uncomfortable with editing registry then here is small app Taskbar Thumbnail Tweaker which provides nice GUI to tweak and resize taskbar thumbnail size or speedup the thumbnail preview, you can easily change the setting using Taskbar Thumbnail Tweaker.
http://http//www.thewindowsclub.com/how-to-increase-taskbar-thumbnail-preview-size-in-windows-7
Taskbar Thumbnail Tweaker for Windows 7 to Resize/Speedup Thumbnail Preview