The slowness of Phosh is purely a consequence of the current, unoptimized software state. When I run Waydroid on PureOS on my Librem 5, it feels much smoother, despite being another layer on top of the OS.

I bet the vast majority of that slowness people complain about comes purely from GTK renderers. A motivated contributor could do wonders there.

Modern GTK uses a lot of embedded JavaScript and CSS. Unless I come across strong evidence otherwise, that's where I place the blame.