I did a quick test and found that it's easy and worthwhile to use LLVM ar instead of GNU ar when building MAME on Windows with gcc.
To make it work, install the llvm package - in my case this was automatically installed by using "pacman -Syu", but your mileage may vary.
Once installed, add AR=llvm-ar to your build command line and you're done.
Preliminary testing reduced the time to build pacman on my system from 14 minutes to 9 minutes; a significant portion of the delta appeared to while archiving libemu.a. I assume the saving will be magnified for a full build, and including symbols might also have an impact, but I have not confirmed this with any testing yet.