Why do sprouted lentils take longer to cook than soaked lentils? I like sprouts, and I thought it would be a nice experiment to sprout my lentils before turning them into a tender soup, as I often do. I usually soak lentils for half a day, and then boil them with various spices for 30 minutes to an hour, until tender and soupy. So I expected that, since germination kind of pre-digests the organic compounds in the seed, the cooking times would be even shorter.

I had to cook them for 90 minutes to get them to be al dente, and I guess it would have taken them at least two hours to get soft. Has anyone had a similar experience with sprout, or does anyone know why this happens? My sprouting time was two days and a half, after the initial soak.
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.