Some users have reported slow or lagging scrolling in Safari 5.0. In some cases, it appears that this issue is caused by the installation of Flash 10.1, which includes several security fixes but may degrade performance in some regards.
Fix
The fix for this issue is generally to downgrade Flash to the previous iteration (10.0), although this will eliminate a number of important security refinements and other enhancements. As such, you should avoid the downgrade if you can live with the slow scrolling.
To downgrade, simply run the Flash uninstaller then install the old version of Flash.