Honestly it's pretty impressive. I'm a little behind the curve when it comes to what we can do now with web publishing and mobile play? I was blown outta the water when I figured out I can publish games that can be played in-browser exclusively. I need to do some more tweaking so that stuff works on mobile similarly, I wasn't really building for it and I wasn't aware that MV's default inclusion of two formats for all their audio assets was because one is for mobile (though that all makes more sense now, I guess it's an optimization thing). Just come from an era where RPG Maker made .exes and that's it XD
Reverse engineering from an existing shader is probably the way to go, I'd imagine. Especially if it's getting as janky as you say. I got the impression somewhere that mobile optimization is being prioritized, so it would make sense to me if they dropped the flexibility to use GLSL for more efficiency? Though I don't really know enough to say how much chunkiness you strip out dropping that support, hmm.