You didn't understand the goal of clamp. clampx(val,min,max) is a function that will return :
val if min<val<mx
min if val < min,
max if val > max
By changing like you did, you set the minimum progress to 0.5, so the movement was half done from the start.
I modified your last capx to include a textbox for you to change the speed on the go, and see how it works. On my computer, you could clearly see the difference between 0.1 and 10 for example.