an almost endless number of things that can affect the performance of streaming
are you connected to the TV wirelessly?
Do you have a Roku or "other" box that has some memory that can buffer?
If not many times rebooting the TV can free up the limited memory of the Smart TV
do you run speed tests when you experience these issues to see if it's the ISP limitation?
Are you fed from the ISP with Fiber?
Obviously, it could simply be the youtube server that can't keep up