To those still wondering what kind of performance improvement Meson brings to an established project over Autotools, here are some numbers from the Yocto project after they switched their native GTK3 build recipe:
config (autotools): 38.5s
config (meson): 17.9s (-53.5%)
install (autotools): 11.2s
install (meson): 1.6s (-85.5%)
compile (autotools): 100.8s
compile (meson): 37.5s (-62.8%)
The build went from over 2 minutes to sub-1 minute