although they can achieve higher speeds than a helicopter since the propeller blade isn't advancing in the direction of travel.

These (and other) problems limit the practical speed of a conventional helicopter to around 160 - 200 kn. At the extreme, the theoretical top speed for a rotary winged aircraft is about 225 kn, just above the current official speed record for a conventional helicopter held by a Westland Lynx, which flew at 400 km/h in 1986 where its blade tips were nearly Mach 1.