The time-consuming part is not building MPV itself, but the long list of dependencies requiring a rebuild in order to fully satisfy the deployment target version. Setting MACOSX_DEPLOYMENT_TARGET in the immediate environment doesn't always propagate, resulting in target mix-ups and build failures for some dependencies. The only working solution I've found is to enforce within the formulas themselves. The following script adjusts the bulk of this:
#!/bin/sh
path='/path/to/homebrew/Library/Taps/homebrew/homebrew-core/Formula'
for dep in pkg-config makedepend autoconf automake openssl [email protected] libpng jpeg libtiff youtube-dl little-cms2 gettext libffi pcre libtool freetype fontconfig cairo pixman graphite2 icu4c harfbuzz glib libogg libvorbis flac libsamplerate libsndfile rubberband theora nasm readline sqlite gdbm xz python@2 python gobject-introspection docutils fribidi texi2html libass ffmpeg mujs mpv
do
sed -i.bu $'s/def install/def install\\n ENV["MACOSX_DEPLOYMENT_TARGET"] = "10.