The phone will often run a bit slower the first few days after an update because the device is still settling. The software must in some ways reconfigure itself, recompile its data, and reestablish all of its links with other apps. The phone is working pretty hard, which is why it seems that battery life takes a temporary nosedive the first few days. Your apps may appear to run slowly as well. Some may need to be updated themselves.
Often, the fix for most of the issues that come with a major update is to perform the factory data reset. That will wipe the device and make it like it just came from the box, except that it has the most recent firmware. Set it up like a new phone and give it a chance.
If you want to make your device look like it did on Android 6, try a third party launcher. You can change your wallpaper, parts of the lock screen, display fonts, icons, the app drawers, home screens, etc. You can make it look like how you want, and save the layout. The next time your phone updates, or you buy a new phone, you can apply the saved layout and all will be as you saved it, even on another device. There are a lot of good ones in the Play Store. Three of the best are Nova Launcher, Action Launcher, Google Launcher. All of them are free, but you can buy a pro version of Nova Launcher and Action Launcher that adds features. The Google Launcher is going to be very similar to what you'd get with "pure" Android (what you'd find on a Pixel or a Moto device). Try them out and buy the one you like -- they're less than $5. You won't be disappointed.
The other thing you can do to change the appearance is to load themes. You're probably limited to loading themes from the Samsung Galaxy Apps app on your phone. But you can change wallpapers, menu colors and fonts, etc. Again, customizing is the key here. Use a theme with a custom launcher and get the best of both.