Some of us still stuck with a first generation Raspberry Pi Zero W really wish it defaulted to its 1 GHz turbo speed, but it doesn't. Under normal circumstances, a Pi Zero W will default to 700 MHz. The fix is simple: add the following line to /boot/config.txt and reboot:
force_turbo=1
That's it.
At least until the Raspberry Pi Zero W v2 is actually shipping...