The truth is, you have no idea what the speed is. It could be 750 mph or 1000 mph, for all you know.
Are you suggesting there is a mass cover up of actual airlcraft speeds? All the airlines, all the engineers, all the air traffic controllers and aircraft manufacturers on the planet are all conspiring to cover up the fact that the planes all travel twice as fast as they say?
You need to get out more.
1000 mph, for all you know.
I live below the busiest airspace in the world - I think I would notice if all the planes were breaking the sound barrier.