My original Fire TV still runs Kodi fine. The only buffering I ever run into is due to the stream I'm pulling or if I haven't run it in a long while and there is updates happening on the backend either in Kodi or the Fire TV. Otherwise it runs really well to be honest.
Had some issues with my Fire Stick lately so reset it to factory and just have it plugged directly into my tv's USB port so it's always rebooted when I start the tv. Takes a minute for it to start up but runs well once fully up. Guessing the newer second gen will run that much nicer.