A bit of an addendum: I think the limiting factor of a mobile keyboard is going to be that you have 1-2 fingers available for use rather than all 10.
A lot of those alternative keyboards seem to rely on some swiping gestures or drawing symbols. That’s good when you’re not looking at it as it’s very tolerant to misalignment, but if you already move fast and precisely, that feels like it would get in the way.
Looking at my fingers while I type, I’m already moving my thumbs as fast as I can without getting too sore: where it goes next doesn’t matter, it’s the same amount of time. Having to press and then move and release, in theory should only slow me down because by the time I finish the drag motion and lift the finger, I would have just moved my finger over the next key and pressed it. So that kind of layering is out of the window.
I can’t think of a way to type faster without involving mnemonics and chorded entry, but with two fingers you don’t gain that much.
Also I feel like the bottleneck there is how fast I can think of what to say next anyway.