We go round and round on this issue and I don't know why, the empirical answer is right in front of you.
Original approximate f/5 - fluorite
2nd Run approximately f/7 - FPL 53
So even Roland, when making the more challenging optical configuration used Fluorite for the faster optic.
Of course all the "failed scientists", and that certain group of converts, are going to run in and go off in all directions (Abbe this and mating element that - blah blah blah) trying to negate this difference, but it speaks for itself.