Many quicks in the past have been capable of short, hostile spells. Who knows how quick Larwood was in the bodyline series? It was enough to unsettle the greatest player of all time. Frank Tyson destroyed Australia, down under in 54-55, with lightning speed. Seasoned pro's like Keith Miller said he was a yard quicker than anything he faced in his whole career. Alan Ward of Derbyshire broke into the England team bowling incredibly quickly but his action couldn't be sustained and injury finished his career. If you watched a fired-up Devon Malcolm destroy South Africa with his nine wicket haul, no-one could persuade me that some of that stuff wasn't well over 90mph.
Now its recorded. Then, it largely wasn't. You relied on the batsmen and keeper/slips telling how quick someone was.
Many bowlers are capable of a 'quick one ' They have it in their locker. Although not English ( I use it as comparison ) Charlie Griffith used to lumber in and generally be slower than Wes Hall. Apart from when he ' threw ' in his quicker one and that went through batsmen before they had brought the bat down! Some quicker bowlers can be more hostile than out and out quick, if you get my meaning.
It is the ability to sustain higher speeds( 90mph+) over a few overs that sets the genuine quicks apart.
I’ve often wondered if there was a way of watching old film and working out how quick they were in days gone by. Surely they could do it couldn’t they ?