Still running pihole on the pi 3b+ after many years and was wondering about the above question since fiber is coming to my area soon ish.
From searches on Google it seems that it wont but what about if I start running other services on the pi? Such as VPN?
If I understand correctly Pi hole works as a DNS service so I don't think the bandwidth (100mb) is an issue.
You're not routing all the network data through the Pi.
The issue is wether the Pi can run fast enough to process all your lookup requests.
So latency in the network would only be an issue if I processed so many requests that I used the entire bandwidth of the port on the pi?
Latency in this case would be the time it takes for your device to ask the raspberry pi what IP address is www.google.com and the pi will reply to the device with the IP address.
I think you might be misunderstanding how the pi hole blocks adverts. It doesn't filter http traffic by looking at the contents but instead has a block list of domains that server adverts. When a device on your network requests the IP address for a blocked domain PiHole just replies with a fake IP (or nothing, I'm not sure)
This link mentions typical request is 512 bytes or less. https://pi-hole.net/blog/2017/05/24/how-much-traffic-can-pi-hole-handle/
This link mentions 250 uses on a single pi hole. https://www.reddit.com/r/pihole/comments/5jpavc/how_many_users_can_pihole_handle_with_a_raspi_3/
Pihole/dns wont realistically be affected (DNS is a tiny amount of traffic). But running a VPN ran on the pi will be limited to the Pis max performance. As long as your CPU isnt getting pegged at 100% or thermal throttling other services shouldnt impact pihole performance significantly.