Most web developers. Webpack + typescript compiling and bundling all the assets.

You want to see a preview as quickly as possible. Best if you can keep a live view on a second monitor.

I doubt much of this is parallelised. Would love to be wrong though!

Cannot speak for everyone, but the builds of my team are heavily parallelized, and we aren't even running a monorepo. For any non-trivial project that has its own somewhat complex build pipeline with many internal dependencies and such, it is pretty much a given.

The only scenario I can foresee where people wouldn't be doing parallel builds is if they are just working on their personal side project with no other people involved, where the need for parallel builds is non-existent either way. You don't want to have one giant monolith project for your whole complex web app, it becomes a nightmare very quickly.

is any of that compute-bound? I would imagine that it's mostly I/O-bound.

My CPU definitely spikes when compiling JS.

Any decently sized C++ or Swift code base is going to have compile times way over 5s. Many just moderately sized projects have build times over 10 minutes.

My current codebase is 28 hours for a full build on one machine...

I dread having to update the compiler or some core "used nearly everywhere" header file, and wait a whole another 28 hours...

This is for a full rebuild, right? If you're changing a few files it shouldn't have to recompile + relink everything.

For moderate to large projects, link time can be a huge factor even for incremental builds. It might be that faster IO is the better investment, though.