DIAGNOSIS The main cause is a full or nearly-full internal storage. Fire OS and apps cache a lot of data, and when storage is critically low, everything bogs down.
FIX First, clear the cache. Go to Settings > Applications > Manage Installed Applications. Go through your biggest apps (like Netflix, Prime Video, Hulu) one by one. Select each, then choose 'Clear cache'. Do NOT select 'Clear data' unless you're okay logging back in. Next, in the same menu, force stop any apps you don't use regularly. Finally, uninstall any apps you truly don't need. Aim to keep at least 1GB of free space.
IF THAT DIDN'T WORK A full power cycle can clear out system glitches. Unplug the TV from the wall for two full minutes. Also, check your network connection. A slow Wi-Fi can make the home screen content load slowly, making it feel laggy. Try a speed test on the TV (Settings > Network).
HEADS UP The Omni QLED has limited storage (often 8GB total, with only about 4-5GB usable). It fills up fast. Regularly clearing app cache is necessary maintenance, not a one-time fix.