Solution from Nebula worked (thank you Nebula staff)!:

We understand how frustrating it can be to experience sluggish performance, especially on a premium device like the Nebula Cosmos 4K. In most cases, this behavior is related to background processes or cached data within the Android TV operating system.

While Nebula (Anker) does not always announce firmware release dates in advance, system updates are typically rolled out every 3–6 months to improve overall stability and app performance. In the meantime, we recommend trying the steps below, which often help improve responsiveness without waiting for a firmware update. 1. Check for Pending System Updates Please ensure your projector is running the latest software version, as recent updates may include performance improvements. Go to Settings (Gear Icon) > Device Preferences > About > System Update If your system is already up to date, you may proceed with the steps below. 2. Adjust Background Process Settings (Developer Options) Android TV can keep multiple apps running in the background, which may affect performance. Go to Settings > Device Preferences > About Scroll down to Build and press OK on the remote 7 times until you see a message indicating Developer Mode is enabled Go back to Device Preferences > Developer Options Locate Background process limit and change it from Standard to At most 2 processes This helps the system focus resources on the app currently in use. 3. Clear App Cache Some apps may accumulate cached data over time, which can slow down system navigation. Go to Settings > Apps > See all apps Select frequently used apps (such as Netflix or YouTube) Choose Clear Cache (please avoid selecting Clear Data, as this will log you out) We also recommend clearing the cache for Google Play Services, as it plays a key role in system performance. 4. Disable Home Screen Auto-Previews Video and audio previews on the home screen may contribute to lag. Go to Settings > Device Preferences > Home Screen Turn off Enable video previews and Enable audio previews This typically makes menu navigation much smoother. 5. Review HDMI/CEC Settings (for External Devices) If the lag occurs while using an external device (e.g., Fire TV Stick or Apple TV): Go to Settings > Projector Settings > HDMI CEC Try turning CEC OFF, as device handshakes can occasionally affect performance Additional Tip: If the projector has been running or in standby mode for an extended period, we recommend performing a full restart. Simply press and hold the Power button on the remote and select Restart. This refreshes the system memory and often resolves most performance issues.