The latest update for Huawei P9 Lite brings some bad news about your camera, because the camera is now very slow when it is started for first the time after a while.
I tried to make a full reset for the phone but with no results. I locked the camera app in background but also any results. Also, it is very important the location where the images are saved. If you use the internal memory of the phone the camera will open a lot more faster in comparison if we choose to save images/ video on the external flash drive (microSD card).
In conclusion, the best solution to make the camera of your Huawei P9 Lite faster is to choose to save images/ videos on the internal memory.
To do that, open Camera App from your phone, slide to left, and unchecked Priority save in SD card.