I have a smaller collection of songs (15,000+) and it takes a few days to integrate them all. I've never been sure exactly what goes on. I run Groove, and it adds songs (one every 10 seconds) until it apparently stops taking songs in. I close Groove, restart it, and it starts importing it again. I would have sworn that it only adds music while it's open, but at some point I'll forget about it, and then a few days later it's integrated everything. I don't know if it would have done it by itself, or if I just happened to forget about it at the point where it was on the last batch of music to integrate.
Regardless, though, I expect it to add all of my music in a much shorter time frame (say, an hour). It's also the same on the desktop, it's slow as can be adding music, whereas Windows Media Player adds it lickety split.