Diagnosis
Accumulated cache from apps and the OS is bogging down the storage, or background apps are consuming RAM. The 3rd Gen Cube has ample performance, so this is a software clutter issue.
How to Fix It
Clear the app caches. Go to Settings > Applications > Manage Installed Applications. Go through your most-used apps (especially large streaming apps like Netflix, Prime Video, Disney+) and select each one, then choose "Clear Cache." Do not select "Clear Data" unless you're okay re-logging into that app.
If That Didn't Work
Perform a full system restart, not just sleep. Go to Settings > My Fire TV > Restart. This clears the active RAM and stops all background processes, giving you a fresh start.
Heads Up
If you've sideloaded many apps, they can run in the background and cause lag. Review sideloaded apps and uninstall any you don't actively use.