having FPs return time increments of 1 or 5 minutes does not change much
you'll always get a group of guests who will clog up the FP entrance 15 minutes before their return time, almost preventing other guests to get through. And they will invariably try to get into the FP line at 3:01 when their return is at 3:05 (and lots of CMs will not bother turning them away, even though some will

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But the FP+ also can create that kind of backup through another phenomenon
Let's say you get a FP+ for attraction A with a return time of 3:00 to 4:00 and attraction B with a return time of 4:00 to 5:00, chances are you'll want to get in line for A as late as possible, so that, when you exit A you can go straight away on B
Those who get to their FP+ return as late as possible will add up to those who get to their FP+ return as soon as possible.
with the legacy FP, I know I would have hit the FP return line as soon as my window opened, because that's basically when I could get another FP (most of the time) and it was rather important to be there as soon as possible.
Now, the strategy is different. Since you can schedule your FP+ you no longer need to be there at the start of your return window
and if you have A, B and C at 3, 4 and 5pm, if you do A as late as possible, B as soon as possible, and C as late as possible, it opens a window (roughly 90 minutes long) for a standby ride in between
in that respect, the 5 minutes increment do not make a difference, because guests no longer return (en masse) at the start of their return window.
(well, I probably should insert a disclaimer too

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