I just realized something kind of strange: I sensed and saw a few mini-second delay when using my Wacom Tablet Graphite 4 with Manga Studio Ex4 under the Windows 7 Final release. I didn't have the same problem when using the Windows 7 release candidate. Odd.
But then you could argue that when you ink your manga, you usually don't do quick sketch and ultra fast strokes. I tried carefully inking, and the delay doesn't seem to affect my inking process.
Maybe I just have to deal with it by inking properly.