Diagnosis
Yeah, this one's common. The processor and memory in the Vidaa U smart platform can get bogged down over time as apps update and cache fills up. It's not broken, just underpowered.
How to Fix It
Don't buy a stick yet. First, do a 'Power Cycle'. Go to Settings > Device Preferences > Power > Power Cycle and confirm. This fully reboots the smart system and clears the app cache without deleting your logins. Then, go to Settings > Device Preferences > Apps. Find the problematic apps (Netflix, Prime Video), select them, and choose 'Clear Cache'. Do NOT select 'Clear Data' unless you're okay with logging in again. Finally, check for a system update (Settings > Device Preferences > About > System Update).
If That Didn't Work
If the interface is still sluggish, do a factory reset on just the smart system. Go to Settings > Device Preferences > Reset > Reset Smart Features. This wipes all app data and logins but leaves your picture and sound settings intact. You'll have to log back into everything, but it's like a fresh start for the smart TV brain.
Heads Up
The internal storage is very limited. Avoid downloading a ton of extra apps. If you use more than 4-5 streaming apps regularly, an external streaming device (Fire Stick, Roku) will almost always be faster and more reliable. It offloads the work from the TV's weak processor.