I am surprised no one else that I can see has brought up the issue with the jerkiness of scrolling through your conversations in Messages on the iPhone 7 Plus. I have had this issue on every iOS version and on both of my iPhone 7 Pluses when I have contact photos enabled and I scroll through my conversations there is extreme jerkiness. I do not remember seeing this issue on my 6S Plus and it does not occur on any of my iPads. I submitted a bug report months ago and have yet to see any change in any beta release.