It's just a self hosted DNS so there shouldn't be any speed reduction (goes straight to Google's DNS if it's not blocked). It very different from ublock origin that reads the html directly from the website. Since it's DNS based, it's not perfect hence why they recommend using pihole + ublock origin. The nice thing about it is that it blocks most ads pretty well (even on your phone) without breaking anything if you use the right host list, and blocks all the telemetry network wide (if you change the DNS on the router).