i6s+ here. I was beta testing versions until 9.2.1. 4 times I had to setup prints: when new, one unintended restore, when become beta tester, when exit beta testing.
Before installing 9.2.1 final, iPhone used to unlock lightning fast. After that I lost phone's overall swiftness. Almost all animations become sluggish. Sometimes even with skipped frames. Unlocking with the prints becomes erratic: I have to try several times.
Only way that works for me is cleanning the reader, "close" all App's (yeah, I know, I know... old bad habit... Or not?), hard reboot. Full responsive phone again BUT fingerprint reader accuracy. Even post unlock animation is fast, but fp reader keeps me asking for prints several times, sometimes at first try, sometimes 3 times... I'm just about to go Apple asking for warranty and finds out what they have to tell me. But first, I'll tray to re-train the prints as some of you have recommended.
Perhaps a combination of both methods helps?