Not stellar. You also have to do some special setup to get SDL to accept the joycons as a gamepad. I would guess it's running at around an average of ~60% speed. But I have no actual numbers. If anyone knows how to get the game to show framerate I'd be happy to report back.
For the record, I was indeed able to build and run it on L4T Ubuntu on the switch. Had to do the comment removal from the sound file as described above and remove "-m32" and change "-march=i686" to "-march=arm64" in the Makefile.