I’m not a programmer at all so I’ll provide the layman’s explanation, for people like me, to accompany Slyp’s well thought out technical explanation 
Basically, with the way the speed settings and animations are implemented for the game in Unity (our game engine), Unity cannot support continuing to run the game at 4x speed while you have a card selected. So it must slow back down in the background while you have a troop selected.
We’ve had players who also happen to be programmers tell us we could fix this and to let them fix it for us if we’re not competent enough (ouch). This one line Slyp said summarises why we don’t always do something just because it can be done:
Game speed interacts completely with our core gameplay. The foundation the entire game sits on. It’s not worth messing around with for a minor quality of life pay off.
I don’t disagree with you, what you’re suggesting might be a nice feature of the game (this is without even touching on whether this is actually an improvement or a nerf to player experience due to visual communication issues it could potentially create), but it’s just not worth the risk vs reward - not even close to being worth it at this time. It has nothing to do with competence levels.
Replace “scientists” with “programmers” here:
PS. I wish I’d come up with this response during the original discussion. Ah, my slow brain.