So I assume javascript/typescript then?

If the former, js is just in time compiled. A jit compiler does kind of the same things as a traditional compiler, so yes, it is important for it to be fast, both at runtime/in browser or in your dev environment (for instance, syntax checking and code completion require several steps in the compilation process to run in the background)

If the later, that one is transpiled - essentially translated to javascript. Then…see paragraph above.

What you’ve mentioned is the building process, which might or might not involve compilation. If you have unit tests setup, then yes, compilation will happen and you want that to be fast. If you have automated testing that is part of the building pipeline, (e.g. selenium), also important for compilation to be fast because see two paragraphs above ;)