No performance hit on being wireless?
nope. I use a Pi Zero W for pihole/VPN duties. The pihole is just a DNS server so the information gets cached on frequently visited sites. The initial connection to a new site might be slightly slower (were talking milliseconds) , but after that it is cached and has no effect on speed. If anything it is faster because it doesn't have to leave the network for DNS requests.
The only issue I've had is when the pihole goes down and is the only DNS, then traffic doesn't no where to go and you'll get no internet. Just make sure in your router you set a secondary DNS address in case the pihole fails then traffic will still route to the second DNS. I use 1.1.1.1 since fuck google.