I ripped out webpacker and replaced it with esbuild in a big legacy rails app for the front end, probably 2-3 years ago, and its been fantastic and I haven't looked back. It's more or less made front end bundling an afterthought. Going from 3s to 1.5s on my M2 (esbuild to mako) isn't a gamechanger, so for me it feels like it's already getting close to the peak, whatever that might mean.
But I was more just asking what's the theoretical limit for this kind of optimization, and at the very least with rust. O(n)?
A that would be hard to say. A lower limit would be reading your entire project and the dependencies that are used once, and writing the bundled/minified code once. Possibly some parts of that could be done at the same time as you determine the bounds of the dependencies as you read in the code. So O(n) where n is in operations over lines of code in your project at least.
There's probably trade-offs too. Like do you bother with tree shaking to make your end product smaller, or do you not to make your build performance closer to that optimal read-once write-once lower bound.
But I was more just asking what's the theoretical limit for this kind of optimization, and at the very least with rust. O(n)?