Fighting is a merciless game. Nearly every fighter is one beatdown away from a rapid decline. So who fell the farthest and fastest in one fight?
Rory Mac v Ruthless?
T-Ferg v Gaehtje?
R3 vs Holm?
Silva v Weidman?
Love the thread and your examples, but have to question a few:
3rd one (Ronda/Holm) Ronda didn't necessarily rapidly decline post-fight; we don't have any realistic sample size to prove it. She got destroyed mentally by losing to the point where she took her ball and went home until she got spoon-fed an opportunity that was probably tied into her getting into Pro Wrestling.
Against Nunes she fought like a moron like she always did expecting her opponents to be scared and got clocked so hard in the first 10 seconds that she was fighting on pure instinct after that. She actually did better then I thought she would in a couple spots but she didn't have power/experience/technique to overcome Nunes blitzing her in the first minute of the fight because she got rocked. She actually show-cased a hell of chin if anything, so for me it's tough to that she truly declined physically/mentally to an extreme degree, she just fought the dumbest fight possible, which she almost always did but her opponents weren't very dangerous and/or skilled to hurt her in those spots prior.
I say this as having won my largest ever MMA bet by getting Nunes as a +160 underdog on this fight
4th one (Silva/Weidman) Anderson wasn't really that shot afterwords (at least Mentally).
He had lost a big step due to the leg break, but he still had a competitive loss against Bisping on his title run (could've won by TKO with a different ref with the flying knee at the end of the round where he did the walk off), got a gift decision against Brunson (definitely lost but was still taking damage and hanging in the pocket), and got a passing of the torch fight against Izzy and lost a decision that wasn't embarrassing.
To breakdown your other examples (perfect ones) I'd say:
Rory vs. Robbie = Rory changed the most in how he fought after (scared to take damage, turned into a pure grappler where he used to be a solid stand-up fighter).
Ferguson vs. Gaethje = Ferguson changed the most in how he physically degraded - still tried to fight like he used to but his body wouldn't let him, it was like he was starting to decline before the fight and then aged a decade overnight.