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In hindsight I can remember similar problems with a Bester 2 k stone. Small pimples of stone seemed to raise up at random. Might be a fault in the manufacture.

What grit paper are you lapping with & how hard are you pushing on the stone ?
I wonder if the particles are from the paper getting embedded. Lapping the stone with a diamond plate would show if...

Learning the stone & the light pressure needed always speeds up the process.
IME the Kings all got faster after years of use & lottsa practice. Nonetheless they are a relatively slow, soft stone...

Just something to check.
If that is literally a few hours on any King stone. I would suggest that stone may be "dishing" ie concaving its surface as you hone.
Keep it lapped flat.

I used to...

Hate to say I toldja so :D

Steve, the trouble with the King nagura is that they become more crumbly as they age if used infrequently but even new ones require a very light touch. I can't see them as being the same as the 8k...

They also say it can be used to flatten stones... I'd like to hear from someone who uses a 12k stone for flattening out high spots. That just doesn't gel with me.

The other thing I mentioned is...

:hmmm: I think any stone that comes with a synthetic is just for cleaning & resurfacing. 12k for such a stone makes no sense :shrug:

Good idea. The slurry stones that come with the King 8k tend to crumble easily & can degrade an edge initially. May be the culprit if you have one of these.

There is also the chance that the 6k & 10k are indeed polishing very well & revealing scratches from earlier stones.

If the shave from your 4k vs 10k is no different there is something wrong. ...