I don't think I could have had a worst experience. All my apps/video start jittering and lagging when I change the volume during playback . It gets really bad . Casting also is very hit or miss . I cast Telemundo Deportes a lot for soccer and at random the picture will freeze and start playing in a PiP type mode . It's really odd . Casting videos from websites is also very hit or miss . I'm pretty sure it's the Sony as I don't have any issues with my chrome cast but the chromecast can't do 4K . I've tried resetting it too because I've had random software issues with ATV, such as repeated apps .
Enable Developer Mode, and force the TV to run 4 apps in the background maximum. Helped a lot with smoothing out any stuttering I had. Restart the TV after you do it, and give it 5 minutes to settle before using it. After that, should be all good. Let me know how that goes.
To enable developer mode, find build number under the about section in settings, and double click on it 4 or 5 times.