I'm not sure about that; the Russian Ka-50 coaxial attack helicopter seems reasonably quick, certainly not the fastest helicopter ever but comparable to an Apache at least (315km/h vs 293 km/h respectively, though the Apache has a higher never-exceed speed: 350km/h vs 365km/h)
If you include compound helicopters (using horizontal pusher props or turbojets), coaxial helicopters can certainly be very quick; the S-97 can cruise at 410 km/h, but in that case you have the liabilities of a 'tail rotor' to contend with again. The S-69 could do a blistering 487km/h with it's turbojets, 289km/h without.
One to consider is the Eurocopter X3, a single rotor compound helicopter without a tail rotor. It could do 472km/h and used two tractor propellers with variable pitch to counteract the rotor torque.
If you include compound helicopters (using horizontal pusher props or turbojets), coaxial helicopters can certainly be very quick; the S-97 can cruise at 410 km/h, but in that case you have the liabilities of a 'tail rotor' to contend with again. The S-69 could do a blistering 487km/h with it's turbojets, 289km/h without.
One to consider is the Eurocopter X3, a single rotor compound helicopter without a tail rotor. It could do 472km/h and used two tractor propellers with variable pitch to counteract the rotor torque.