Why do sprouted lentils take longer to cook than soaked lentils?
The biochemistry of the sprouting process is a little beyond me, but essentially the lentil converts stored energy (carbs) and protein into different proteins and fibre.

The new proteins are quite useful to our own biochemistry, which is why sprouted lentils are considered "better" for you. 

It is the fibre, however, which causes the cooking time to be longer. 
Generally, whenever something takes a long time to cook, it's because it has a lot of fibre in it. Conversely, fast food chains take as much fibre as possible out of the food, to shorten cook times.