Diagnosis
The internal storage is nearly full, causing system lag, or background apps and cached data from automatic updates are consuming all available RAM.
How to Fix It
Clear the system cache and force stop bloated apps. Go to Settings > Device Preferences > Storage. Check available space. If below 2GB, go to Apps, select large streaming apps (Netflix, Disney+, Prime Video), and Clear Cache and Clear Data for each. Then, go to Settings > Device Preferences > About > Click on 'Build Number' 7 times to enable Developer Options. Inside Developer Options, set 'Background process limit' to 'At most 2 processes'. Reboot.
If That Didn't Work
Perform a soft factory reset. Go to Settings > Device Preferences > Reset > Erase all data (factory reset). This will not affect the underlying projector firmware (light engine settings, keystone), only the Android TV OS and installed apps. Re-login to all services afterward.
Heads Up
The Horizon Ultra's Android TV OS runs on a limited resource budget. Avoid side-loading numerous apps or using it as a general web browser. For best performance, use an external streaming device (Apple TV, Shield) via HDMI and treat the built-in OS as a backup.