From the home screen, try pressing these remote buttons in quick succession:
Home 5 times
Up arrow
Rewind 2 times
Fast Fwd 2 times
This forces a cache-clear and reboot, which can help restore performance on a Roku that's become sluggish. Some of the manufacturers unfortunately skimp on memory with their smaller sets. I have to do this sometimes on our kitchen TV (TCL 3-series 32s327) after using one of the more resource-hungry apps like Netflix or especially Amazon Prime Video. Even with limited RAM & flash, the Roku OS really ought to be able to clean up after these piggish apps on its own, and hopefully a future update will make improvements in this area, but at at least the manual flush is available until then, and only takes about a minute to run.
