The problem with speed differences come from different crews/aircraft choosing to lose time in different ways - if the front one reefs it right back early the back can close up remarkably quickly.
Seen that a few times. There are a couple of ways to skin the cat; one of them results in exactly that. Tigermoth cruise speed and Concorde shuttle descent speed...
Can we program Maestro to give us another 15, 20 mins to slow down
Agree. In the "old" days when humans did it, we got our FF time nice and early (even in the climb!) and didn't need to climb to the moon, only to descend back down almost straight away. Now the machine's doing it, we get our FF time much later which requires more drastic action. That said, over here the ATCs are getting adept at dropping "hints" about how much time we'll have to lose instead of just saying "reduce by 0.05", which in the big scheme of things has very little effect at all.