Did you try using "device care" in the settings of the TV? Also there, you can perform diagnostics tests on your TV. You should do a speed test of your internet. Slow or poor internet connection can cause the lag.
Hey, I can only imagine the frustration of dealing with this. Since it looks like you've tried all of the recommended troubleshooting, I recommend following MrGhoul advice by letting a service technician look this over. As we would like to look at this from a case by case basis, we would need to gather additional information. Please provide the full model code and serial number of the device as well as your best contact phone number, name, and email.
