Maybe, they have lots of devices for repairing at moment. So, they take long time to fix iPhone 6s screen.
If you want to fix iPhone 6S Screen under 10 Min. Must watch this video: https://www.youtube.com/watch?v=fxmuHO_LI5k
My best guess here is that there's a long queue of devices to be repaired and/or they have insufficient parts at the moment.