Yes Physics. As 13 yards is further than 6 yards, it naturally takes longer for a WR to run a 13 yard route than a 6 yard route with some exceptions.
If you dont understand why then I suggest you discuss with your 3rd grade teacher why she/he didnt adequately prepare you for life. I believe you mentioned a son so please make sure he has better teachers.
That's nothing to do with WR separation, separation is based on the WR vs the Defender, and the play design, running deep just means the play takes longer to develop, but that doesn't mean the WR is open more or less.
This is the 2nd time I have had to help you with your own counter point. Now again I'm not doing the work for you, you brought up separation, so you need to show that the separation was worse with Fields vs Trevor.