This is why it's a good idea to not use the actual apps built into smart TVs. They have slow clunky processors with not enough ram and the performance is terrible.
You are much better off ignoring the built in apps and connecting a streaming player to one of the HDMI inputs instead, you will get a much better performance.
I think when it comes to Roku TVs people are fooled into thinking that the software and app experience is going to be just as good as using a high end Roku player like the Roku Ultra, it isn't.