This is too long for a comment.
I believe there is a flawed premise in your question:
on modern (and even not so modern) computers, it would be trivial to output hundreds of megabytes per second using ChaCha8 or AES-CTR
You are thinking of a desktop browser on an AC connected machine or a laptop with a big honkin 10Ah battery.
We live in an increasingly mobile-first world, and while mobile devices these days are quite powerful they have two important constraints: heat and battery life. Unlike a desktop processor which can easily reach 100C, you can't burn the hand off the smartphone user. And phone batteries typically only hold maybe 1/3 as much as a laptop (if you're lucky). There's simply no good reason to add the extra heat-generation/power-consumption if you don't need it.
