A couple of things pinkl. Smooth all the chips off the stones edge. You don't want a tentative stroke to compensate. It will be counterproductive & your strokes need to be spot on not "sucky" .
If it is indeed a slow stone use it as such & have the blade as good as possible before using it. Remember your shaves should be pretty great at 8k. I have a feeling you may need to do more intermediate work before going to the finisher.
Also 30 -50 strokes is not necessarily slow but your C12k may actually be fast.
As for the Sheffields I do very minimal strokes on my J-Nat finishers. Generally I find the Coticule gets the best out of the old wedges.