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?
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.