I wonder if as people get on the NBN (ill-fated fibre (now copper) broadband project) whether the Pi will be a bottleneck. I can download at ~90 megabit on a good day - that's about 3 times faster than my Pi 3 can handle.
OP here. Pi-hole only resolves DNS, your throughput will not be affected. Actually, it makes your Internet browsing faster as Pi-hole caches your DNS requests. We run Pi-hole on Raspberry Pi 1 & NBN and it just works fine.
You won't notice any performance loss as long as you choose DNS servers which are relatively close to you (low latency). For example your own ISP's servers should do fine.