If Hasnain is selected then from Pak he will probably be the fastest unless

finds his fire against Ind.
From Ind. I think it will be Bumrah who in my opinion is the only real contender from India for fastest bowler spot in WC.
From Aus unless Starc finds really good rhythm again it has to be Cummins. If Starc, Cummins, and Hazlewood stay fit for the majority of the tournament Aus can perform pretty well.
From SA Rabada? Lungi Ngidi is also pretty quick because young years and all but Rabada has the real pace in my honest opinion. Steyn might have a good tourney like last time but nowhere near his pace 4 years ago especially after coming back few months back from being injured for over a year.
For NZ it's probably Ferguson with Boult providing Back up, Southee never really had the pace and also got injured late last year so not sure if he will be in good form for the WC.
For BD, I haven't been following them lately so no idea. Maybe the Fizz?
From the Windies has to be either Taylor or Alzarri Joseph. Could be either one of them as both have found their mojo in the ENG series so will be exciting to see them.
From ENG, I don't know. Unless Jofra Archer is selected then no Englishman really threatens any of the other fast bowlers in the tournament. I guess Tom Curran will be one of their go to pacemen in the tournament but he barely touches 140 on his day and I think something like 30% of the balls he's bowled in the last 18 months have been slower balls so it's not like he's striving for pace. England might pull a surprise selection with Mark Wood after he also found some pace and form in the Caribbean in which case it will be interesting to see how he performs.
Who am I forgetting?
SL will probably have Lakmal as his spearhead but they don't seem to have the pace bowling stocks or the bench strength to build a strong pace battery. They have decent seamers who on their day can win them matches but no out and out fast bowler who I am aware of. Correct me if I am wrong please.
So yeah, personally I think that with Hasnain being young, raw, and bowling with real heat in PSL, he might be one of the quickest, possibly crossing 150kph again on a day for him.
Otherwise it's either Starc, Cummins, Archer or Wood (if either is selected) one of the Windians, and possibly Rabada. Bumrah can also sneak through.