For release but not for development.
Sufficient for the build step to take a long time and you start to notice the friction.
The web/browser should not rely on bundlers and compilation steps overall. This should remain optional.
If you've got a huge project, even very quick bundlers will end up slowing down considerably (although hot reload should still be pretty quick because it still just affects individual files). But in general, bundlers are pretty damn quick these days, and getting even quicker. And of course, they're still fully optional, even for a framework like React.