The Disk Cleanup utility that comes as a part of Windows has an annoying feature. As a part of its scan procedure, it tries to figure out how much space you'd save by "compressing old files." This step takes a ridiculously long time to complete, and is highly annoying. Thankfully, disabling this feature is simple, though it involves editing your Windows registry. As always, be very careful during the editing process.
To disable the "Compress Old Files" operation, navigate to this registry key, and delete it:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\VolumeCaches\Compress old files
Once you've deleted the above key, start up the Disk Cleanup utility and marvel at how much faster it loads!