How fast did airplanes go in ww1?

Basically they flew between 80 and 120 miles per hour as top speeds. This varied with the design, amount of drag in the design and the engine used. As the war progressed so did the technology of the aircraft.

At the on set of the war and as planes started to be used their top speeds were around 80 mph for fighters such as the Neuport 11 and Fokker Eindecker. Observer/bomber aircraft were slower due to size and payloads.

At the end of the war, aircraft such as the Spad XIII and Fokekr D.VII had top speeds of around 120 mph, both were fighter aircraft with inline water cooled engines where as the Neuport 11 and Fokker Eindecker had rotorary engines with less horsepower.

Drag is what holds the plane back and in WWI drag was associated with the rigging wires used to strengthen the wings.