>but mobile devices also do not necessarily need AV1 encode support anyway

I would highly, highly argue that they do. There is no reason in modern day that we need to all be uploading the full size of images and videos to servers to be re-compressed. It creates a significant barrier to entry for any social media platform when it should be as simple as the device itself being able to encode it, with the server checking it and creating other sizes. If AV1 becomes the great standard we can reduce a lot of costs. We already could do it with jpeg compression, but hasn't seemed to happen.

Slightly less compression rates for faster compression is perfectly acceptable. I would hope theres profiling options for that.