Diagnosis
The internal storage is likely getting full, which bogs down the processor. Smart TVs, just like phones, slow down when cache and app data pile up.
How to Fix It
You need to clear the cache. 1) Go to Settings > Apps > See all apps. 2) Scroll to and select "Google TV" (it might be called "Android TV Home" or similar). 3) Select "Force stop," then select "Clear cache." Do NOT select "Clear data" or you'll lose your home screen layout. 4) Go back and repeat steps 2-3 for the "Google Play Services" app. Restart the TV from the settings menu.
If That Didn't Work
Check for and install any pending system updates (Settings > System > About > System update). If it's still bad, a factory reset is the nuclear option. Back up any login info first, as this wipes everything.
Heads Up
Avoid filling the TV with dozens of apps you don't use. Each one runs background processes. Stick to the streaming apps you actually need for the snappiest experience.