Es ist ein neuer Hotfix für Revit 2012 verfügbar, hier der Link
Hotfix – Autodesk Revit 2012 – Sketch Grid Performance
This hotfix improves performance when utilizing Sketch Grids within your project.
Make sure that Update Release 2 has been applied prior to adding the hotfix. Verify that the build number of Revit 2012 is 20110916_2132. If the build number is 20120126_0600, it indicates that Update Release 2 and the Large Dataset Stability Hotfix has been applied