Diagnosis
The processor in the SmartCast system can get bogged down over time with cached data and background processes, especially after updates. It's a common complaint with this platform.
How to Fix It
Don't just turn the TV off. Go to Menu > Admin & Privacy > Soft Power Cycle. This fully reboots the smart system. If it's still slow, do a 'Reset TV Audio & Video Settings' from the same menu. This clears all your picture modes but often speeds up the interface without a full factory reset.
If That Didn't Work
The nuclear option is a full factory reset. Menu > Admin & Privacy > Reset & Admin > Reset to Factory Settings. You'll have to log back into all your apps, but it's like a fresh start for the sluggish brain of the TV.
Heads Up
Avoid using the TV's built-in apps for demanding 4K HDR streams if the lag bothers you. A $50 external streaming device (Roku, Fire Stick) has a faster processor and will give you a much snappier experience than the built-in SmartCast.