The masked tip size is slightly less than 100x100px. If I decrease the size parameter (which is around 350 by default) to 80, it is already much faster.
It does look a little more natural on blending these greys, but the performance dip doesn’t offset it’s nicer appearance when overlaying strokes for me.
(left is the one from the bundle with normal blending mode, right is the multiply mode, same colours for both).
E: I think it’s because I messed with the masking, but the multiply marker looks more worn. Also, it doesn’t accumulate ink as much on the stroke so it doesn’t have this nice wet feel that the other one had.