It takes a moment to get the speed for a sprint. And yeah, if you run out of stamina you need to wait a bit.
Shields are raised and then, after a brief moment, put into a blocking position. In DK2, you're committing to a block or being hit; there is no more ninja-blocking. I've got no idea if Agility affects the speed of this or whether it is shield specific.
I haven't found shields very useful in DK2 and I think these changes are part of it.
Actually you don't need to wait. Roll once and you can magically sprint again. Of course, you won't be able to sprint for long but it does mean that you don't have to wait for the bar to refill at all.