iphone 6s camera lag

I’m experiencing an issue with my iPhone 6s where the camera app is sometimes very laggy when starting up. Most of the time things are fine – I open the camera app and instantly see images from the rear camera. However, on occasion I will get either the last photo taken showing or (more often) see really slow updates (like a frame a second) from the camera for 5-6 seconds before it becomes usable. I haven’t seen anything online about others experiencing this issue, so it would be good to know if this is an isolated hardware issue or something others are seeing (and likely to be fixed by a future software update). FWIW I have done a complete restore on the phone via iTunes without the issue being solved.