I don’t want to be that “works for me” guy but I cannot relate to the experience you’re both having. My XS Max runs iOS 12.0 even more smoothly than the 7 Plus it replaces (which was already very smooth) so I feel like there’s something wrong with your install. I’d seriously consider DFU and clean install without restoring a backup. I know that’s very inconvenient but I feel like it would get you to where you want to be, having a smooth experience. If you’ve done that already, I’m sorry, I don’t know why the same device would differ so much.