Speed gun technology has advanced with time, and it’s also difficult to get speed readings from recorded video given the different frame rates, conversion from one format to another, playback speed variation etc.
However one thing is clear that when legends of cricket including Imran Khan who himself was a genuine fast bowler recognised that Holding and Jeff Thompson were the fastest bowlers he had seen then there’s no doubt these guys were express and genuinely quick. The same with other WI quicks like Andy Roberts, Malcolm Marshall, Patterson, from that era, and anyone who witnessed their fast bowling spells as their quickest will have no doubt these guys were lethal.
The fact that these days even an Ishant Sharma or Stuart Broad who look pedestrian to the naked eye in comparison to the genuine quicks of 70s/80s/90s era and yet they are clocking at 90 odd mph now and again gives a hint about what kind of electrifying speeds the likes of Holding, Thomson, Roberts, Marshall we’re bowling at consistently who had wicket keepers standing miles (ok, slight exaggeration) behind wickets.
Let’s face it even Javagal Srinath clocked at 90 odd on the odd speed gun here and there, no one recognised him as a genuine fast bowler even in that era ? He definitely looked like a medium pacer compared to Michale Holding just watching with the naked eye, unless my eyes deceived me.