Bit of a head scratcher here, this is on an i7-4790K (four physical cores), 16GB and a dedicated fast SSD, on Windows 10
MAME compilation for some time has become very slow, over 1h for a SUBTARGET=MESS build with -j9
The build environment is MSYS64 from the current tools. If you look at the statistics for the eight virtual cores, they're all busy but maybe at 10-20% maximum.
Now I have kept an old set of build tools that was manually updated from the 2016 archive ... I tried that today, and all virtual cores are at roughly 90%, the same build is done in just over 20min.
What's happening here?