Ill be honest, I havnt had a TV in about 15 years, so I know very little about this!

My parents TV has a Xiaomi Mi Stick in it, a few years old (TV about 10-15 years old now). Recently, my dad said that when hes been trying to watch TVNZ On Demand, the shows stutter/buffer/lag for about 15mins and then they come right. Is this the Stick getting old? I would have thought that if it were an issue with the wifi, then it would keep stuttering and not come right. I still dont even personally understand how these Sticks/Chromecast work so im at a loss on how to diagnose the issue and fix it.

If its the Stick getting old and we need to replace it, what is best to buy? The main thing is an easy to use UI as my parents arnt tech-savvy, and something a bit faster than the Xiaomi Mi Stick which has always been incredibly slow to navigate the UI/open apps and seems to be getting worse.

Many thanks for any ideas or advice!