I can't do any objective measurements but there's a clear impact in battery life while browsing with Javascript disabled. The entire web becomes snappier and loading times instantly drop.

Doing so also breaks all web applications (and web applications posing as websites, i.e. React rendering) so it's not something I turn on permanently. I usually only disable JS when my phone is running low and I'm not near a charger, or when I'm trying to read something and the terrible website hijacks scrolls/taps/somehow makes my phone run hot doing stuff in the background.