DIAGNOSIS: The built-in storage is likely nearly full from cached app data, which chokes the processor. This is a common smart TV issue, not unique to Hisense, but it really bogs down the interface.
FIX: First, force stop and clear the cache for the Google TV Home app. Go to Settings > Apps > See all apps > Google TV Home. Select 'Force stop', then 'Clear cache'. Do NOT select 'Clear data' or you'll lose your logins. Next, go back and do the same for any apps you use heavily, like Netflix or Disney+. Finally, power cycle the TV: hold the power button on the remote and select 'Restart'.
IF THAT DIDN'T WORK: If it's still sluggish, a factory reset is the nuclear option. Go to Settings > System > About > Reset. This will wipe all your apps and logins, setting the TV back to out-of-box state, so it should run fast again.
HEADS UP: After the reset, be selective about which apps you reinstall. Every app running in the background eats RAM and storage. Stick to the few you actually use daily.