So first I thought "screw lav, let's try ffdshow, always worked before." But playback was still choppy.
Then I remembered "oh yeah I should uncheck all those internal filters to force it to use ffdshow video decoder."
Out of laziness I didn't uncheck every format, just Matroska from the left column, and H.264 from the right.
Nice, smooth playback.
Then on a hunch, I rechecked MKV, removed the ffdshow external filter entirely.
Still nice, smooth playback.
So the culprit is the internal H264/AVC transform filter. Which strikes me as weird... H.264 is the most popular thing on earth. If they get just ONE thing right, just one thing that works correctly right out of the box, it should be H.264.
Here's the setting I mean if it's unclear:
https://i.imgur.com/PtPGegy.jpg