I don't personally use VSFilter nor FFDShow, but the fact of the matter is that all three (i.e. BSPlayer itself, FFDShow and VSFilter) can show subtitles. Are you sure there is no conflict between them which could cause hight CPU-usage?
P.S.:
I would check settings under
Options > Preferences > Subtitles > [Subtitle properties] & [VobSub]
and
Options > Preferences > Filter management > [General]
("Allow intermediate filters" & "Enable advanced graph building" for non-text-based subtitles)