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.