Mine can get laggy if it's been on for a couple of weeks, but a reboot puts it right back to normal.

Honestly I don't think I've ever been as happy/satisfied with a consumer electronics device as I am with the Shield. It does everything I wanted it to do and more. It's a shame there will probably never be another model.

Laggy / stuttering interface may well be a unsuitable HDMI cable…

Yes, really - switching cables instantly got rid of any stuttering in the UI no reboot or setting could fix for me - if I wouldn’t have experienced it myself I probably wouldn’t have believed it..