How can I disable "Top Sites" in Safari/Webkit completely?
There was a way to disable TopSites, but I don’t know if it still works:
defaults write com.apple.Safari DebugSnapshotsUpdatePolicy -int 2
to prevent safari from creating the previews (and using a placeholder).
To undo the above, you need:
defaults delete com.apple.Safari DebugSnapshotsUpdatePolicy
Other than that, there is nothing else you can do.