That's not an easy question to answer. It's probably a combination of things. As Roku's firmware and apps get more and more bloated, it takes more processing power and memory to keep up. An older device probably has less of both than a newer device.
My personal feeling is that all of these tiny boxes and sticks cook themselves over time. I don't have an Express (or Premiere in the same package) device that hasn't shown problems after a few years and I can't be sure, but I feel like they're running a lot hotter than they did when new.
Don't rule out the power supply as the problem. All these cheap USB power adapters also have a short lifespan in my experience. If you're using the TV's USB port for power, try a USB power adapter plugged into the wall. If you're already doing that, try a different one. You might also try changing the display setting to 720p if it's set to 1080p. If you don't notice a difference in the picture or can live with the difference, the lower setting may take a little load off of the device.
P.S. - I treat Express devices as disposable. If you got 5 years out of one, I'd say you did pretty good.
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