Note that it's not a particularly optimized algorithm: recursive descent + specialized subparser for expressions is simply the standard way to write parsers by hand. It's ANTLR which is super flexible but also dog slow.

Yeah, one of the interesting parts to me while working on this is that the breakpoint for when it's worth writing your own parser vs accepting ANTLR's slowness has shifted massively. Previously it would have been someone's full-time job to maintain. Now with this approach you can get the best of both worlds.