Good god...i appreciate the face lifted UI and all but update 9 is really...really bad. Never have I had to deal with so many bugs and app hangs. homescreen crashes randomly and pretty much all my apps glitch out now, even my trusty Fleksy keyboard isn't safe. I tried to send a video on Snapchat a couple nights ago and my phone pretty much completely locked up, PAINFULLY slow. Also I like and hate the new hidden buttons, when you want to switch apps it seems like instead of showing a capture of the app from when it was closed it tries to run it live, this causes issues because it slows down the phone and when you try to swipe away an app (and you can only swipe up) you tend to scroll like you're still in the app, it's frustrating. My headphones don't work properly anymore either, I often cannot go back to the previous song or restart the song by using my klipsch r6bts (which are android compatible). Android 9 is going to need some serious patching and optimization for the Galaxy S8.
MY S8+ (back up phone), and my fiancee's S8 run quite smoothe after the update (I beta test Snapchat also) so just clear cache. If you dont like the navbar then install Good Lock and use Navstar. As for the headphones clear the cache for them (I'm assuming they're bluetooth) and it should be fine. After every update its ALWAYS recommended to wipe the cache partion. Shaggyskunk and others have numerous posts on here so just search it and everything should be fine afterwards. Lastly restart your phone each day just in case you have a carrier or other update that needs a fresh restart. I schedule mine for 5am just before I wake up. Hope this helps!