I have never seen much difference in feathers as far as speed, but have as far as stability in the crazy shots I have to manufacture in the field. I found this in practicing crazy shots in the back yard. As I have stated here MANY times before, I use 5 inch 4 fletch for a variety of reasons.
If I want a faster arrow, I drop Arrow weight (rare occasion for TX) As a 60 grain difference it weight makes MUCH more difference in speed than fletching. ( Dropping to Trad Lites makes my arrow go from 585 grains to 525 grains)
That being said, I've killed animal from 5 yards out to 60(gofers with Doug Campbell) with 5 inch 4 fletch.
I'm sure 3 4" will work on my 'go to' shot, but its not for me, wont work how and when I want it to....but that's just me. I need A different arrangement for my 'bag of tricks'.
Know your trajectory is what is important. Once you know that in consistency....your accuracy will be impeccable.
Just my take.