And designers seem to be far removed from perfomance of smartphones and download speed.

What? Smartphones are faster than computers from a few years ago. Smartphones have been running Fortnite since at least 2018. I really wonder what your designers ask from you that is more demanding than that.

Smartphones are faster but the web apps are still slow or even worse, slower.

Many useless design elements drain the battery and take time to render.

Fortnite isn't a web app and I doubt that Figma was used by the game designers.