I tried this on my MacBook Pro (8 CPU threads) under macOS 26.6.2 using GNU Make 3.81:
make 21.39s user 15.81s system 101% cpu 36.742 total
make -j2 20.99s user 16.01s system 187% cpu 19.748 total
make -j3 22.70s user 17.31s system 245% cpu 16.293 total
make -j4 24.60s user 18.16s system 304% cpu 14.057 total
make -j5 27.17s user 19.88s system 337% cpu 13.925 total
make -j6 27.50s user 20.33s system 370% cpu 12.920 total
make -j7 28.67s user 21.04s system 388% cpu 12.781 total
make -j8 28.95s user 21.24s system 423% cpu 11.847 total
make -j12 29.46s user 21.89s system 403% cpu 12.717 total
make -j16 29.20s user 21.41s system 424% cpu 11.923 total
Each "time make" command was run after a "make clean". Compilation time is divided by a little more than 3. That's nice!
I didn't get any compilation error.
Unfortunately, as expected, the following stayed the same:
make check 39.63s user 51.56s system 52% cpu 2:53.97 total
make -j8 check 40.37s user 52.18s system 51% cpu 2:57.99 total
For me, being able to run tests in parallel would be a big improvement. Doing so is probably non-trivial. Just avoiding garbled output might already be non-trivial. However, a version that runs in parallel with garbled output but a correct summary would already be a great improvement. Usually, when I run all tests, it's just to check whether all tests still pass and, if not, find out which ones have problems.
Philippe