I have a BRAND NEW S9. If I use the camera, after about 10 minutes of taking pictures, the phone lags incredibly bad! It will freeze and eventually become useless. Have to battle to restart the phone so it's functional again. This doesn't happen if I take a couple pictures, but if I'm touring a place and want to take several photos, my phone will completely freeze. It sucks. No apps running in background and it doesn't matter how I have the camera set up. I loved this phone until I came across this problem. Now I hate it.
Take it back if it's new, if it wasn't new I'd do a couple of things like clear the camera app storage and wipe the cache partition but being new I just wouldn't accept it