I recently just set up a second pi-hole as a backup incase the first is out of action for any reason. Went for the cheapest and lowest power option I could (PiZeroW). My initial pi-hole is running on a pi2 model B+.
Before setting up my second pi I was unsure if there was any performance penalty for using WiFi and I couldn’t find an objective answer to this.
Now that I have both running I can say that there is about a 5ms performance hit for using WiFi rather than wired. This is definitely not perceptible in general use.
Rasp Pi 2 B+
Rasp Pi Zero W
For reference the rest of my network runs on unifi.
The wired pi is connected to my main 16port switch and the wireless connected to one of two unifi AC lite APs in the house.