Is your QuickLook painfully slow after upgrading to Mavericks? Until there’s a solution from Apple, if you’re on a mac with a Retina display try this:

1. Open the Applications directory in Finder
2. Right-click the application Preview
3. Select Get Info.
4. Enable/check the Open in Low Resolution.

Source: http://wccftech.com/how-to-fix-quick-look-in-os-x-mavericks/