What the **** happened to mesh lights in 4.20?

I use iRadiance's HDR Mesh Lights vol. 2 for my key lights in pretty much every scene I make. In 4.20 they are completely unusable--the light seems to be a) going in random directions and b) *massively* bogging down render times to the point where I can't even make out the image after 5 minutes on a two-character render (GPU). I'm guessing it has something to do with the volumetric rendering update, but I can't find anything in the Render Settings to fiddle with to try and fix it.

I'm going to have to switch back to 4.19 until whatever this is gets fixed. Anyone else having problems with their favorite lighting rigs?

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