A friend and I forked UE4 a while back and stripped all of the bloat to try and get it running well on a shit-tier Chromebook. We only stripped it down to 30 mb, but the end result (physics + terrain) ran way smoother than any Unity game I've played on that ToasterBook.

Plot twist: people stop writing casual games in Unity and switch to Unreal to target low-end hardware.

Needless to say: as someone who spent my teens and early 20s working on multiplayer web games, I'm incredibly excited for this.