I don't think just building JS is slow, I think the issue is that most of the ecosystem is ready to move on to more "no build" scenarios now that ES modules are supported everywhere that matters [0] [1] (Node 12+). Webpack still doesn't seem to have much of an ESM "native" approach/roadmap. Snowpack had a relatively good ESM "native" approach, but appears finished and points to Vite now. Vite isn't entirely ESM "native" yet, but there's more work there mentioned in these release notes and it definitely appears to be in their roadmap/planning horizon. I'm curiously following Vite, but so far it smells to me more of a webpack too much of the kitchen sink "fork" than not still for the "no build"/"targeted build" world.
I've been experimenting with some stuff using esbuild directly in a "targeted" way that builds only a few things with most stuff just Typescript transpilation only (again). I've almost got a feeling the next winner may just be Typescript+esbuild, but not at all because esbuild is fast or written in Go. It may still be Typescript+rollup, given the hints of what's coming in the next Rollup major. (Typescript is still in JS and getting faster every release, with incremental build support for large projects getting better all the time. Rollup is still in JS and still a decent esbuild competitor.) In a "no build"/"targeted build" world it's not really speed that matters but a minimal, targeted API that's easy to automate as you choose targets to build and it may be the case that the time of the "kitchen sink plus a million plugins" builder is over.
I've been experimenting with some stuff using esbuild directly in a "targeted" way that builds only a few things with most stuff just Typescript transpilation only (again). I've almost got a feeling the next winner may just be Typescript+esbuild, but not at all because esbuild is fast or written in Go. It may still be Typescript+rollup, given the hints of what's coming in the next Rollup major. (Typescript is still in JS and getting faster every release, with incremental build support for large projects getting better all the time. Rollup is still in JS and still a decent esbuild competitor.) In a "no build"/"targeted build" world it's not really speed that matters but a minimal, targeted API that's easy to automate as you choose targets to build and it may be the case that the time of the "kitchen sink plus a million plugins" builder is over.
[0] https://caniuse.com/es6-module
[1] https://caniuse.com/es6-module-dynamic-import