So I have this bug where kernel config A goes fast; kernel config B goes slow.  They are insanely different.  Where to start?

I’m trying this approach.  I diffed the two .configs, then used a  neato tool called splitpatch to break that patch into 100+ smaller patches, one per hunk.  Put the .config into a git repo and applied all the patches (used guilt to import them; probably there is a more straightforward way) and now I’m using git bisect… We’ll see how it goes.  Unfortunately “make oldconfig” wants to modify the .config a bit each time so it won’t be perfect, but hopefully this will get me there.