A smooth exponential scaling on the "homeguard buff" from minute one?

Instead of the flat amount at 20 minutes. (imagine the y=e^x graph with the x and y axis being representative of time and movement speed respectively) Thoughts?

I haven't exactly thought this one through for too long, so I'm not sure of the negatives. Feel free to convince me otherwise because I don't have an opinion on the idea myself.

Also, the potency of the buff would cap after a certain amount of time, say 35 minutes of whatever.