My iPhone is slow after upgrade of iOS

I have an iPhone 6+. It was working great and I resisted upgrading the iOS; I finally gave in because I bought an Apple Watch 3 and it demanded I upgrade. Now, the phone is incredibly slow and laggy - switching between apps, or even getting the home screen after a button press, takes a long time. The worst offender is the camera, since the subject has moved or left by the time the photo actually gets taken. Is there anything I can do - this is pretty ridiculous...