sphinx-build has a -j N option:

Distribute the build over N processes in parallel, to make building on multiprocessor machines more effective. Note that not all parts and not all builders of Sphinx can be parallelized. If auto argument is given, Sphinx uses the number of CPUs as N.

https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-j

Test on a new 10-core Mac

make -C Doc clean venv; time make -C Doc html

Results of two runs before

make -C Doc html  64.83s user 0.84s system 98% cpu 1:06.79 total
make -C Doc html  64.61s user 0.84s system 99% cpu 1:06.04 total

Results of two runs after

make -C Doc html  73.28s user 3.09s system 219% cpu 34.826 total
make -C Doc html  74.05s user 3.09s system 221% cpu 34.791 total

That's nearly twice as fast: 1m06s -> 35s

Test on an old dual-core Mac

Results of one run before

make -C Doc html  204.87s user 5.07s system 94% cpu 3:41.28 total

Results of one runs before

make -C Doc html  278.01s user 13.18s system 196% cpu 2:28.38 total

That's about 1.5x faster: 3m42s -> 2m29s