Taking high resolution pictures needs to process the image data. Based on GPU I, it may take a while, like ~ 2-3 seconds for ~ 14-20 Megapixel camera phone. Some phones camera software may take several pictures to increase picture quality or prevent blurry photos using software, which also increase processing time.
You can manually reduce the photo resolution from camera app settings, which may reduce overall processing time. Turn off HDR as well if not needed.
Also try some 3rd party camera app and try which suits your needs.