I updated from 6.0.2 to 6.0.3 and this is a new bug that has caught me:
Preferences -> Event Display
When the Grid Overlay preference is active, horizontal zooming and scrolling in the project window slow to a CRAWL. It takes 1-2 seconds every iteration.
(However vertical scrolling seems largely unaffected and is almost as fast as it was in 6.0.2.)
As soon as the Grid Overlay Intensity slider is set all the way to the left, zooming and scrolling is fast once again, as it was in 6.0.2.
8-core Mac Pro, SL 10.6.8 64-bit kernel boot, Cubase 6.0.3 64-bit
NVIDIA GeForce GT 120 & NVIDIA GeForce 8800 GT