DIAGNOSIS The remote uses Bluetooth for voice commands and setup, but for basic navigation (d-pad, power, volume) it actually uses Infrared (IR), which requires a direct line of sight. Lag or non-response usually means the IR signal is blocked or the batteries are low.
FIX First, replace the batteries with fresh ones. Weak batteries cause all kinds of weird remote behavior. Then, make sure nothing is physically blocking the IR sensor on the bottom front of the TV. Also, bright sunlight or certain types of LED room lighting can flood the IR receiver and interfere.
IF THAT DIDN'T WORK Re-pair the remote's Bluetooth connection. Hold down the 'Back' and 'Home' buttons together for about 5 seconds until the status light flashes. This can resolve conflicts where the remote is stuck between IR and Bluetooth modes.
HEADS UP If you use a soundbar or other device in front of the TV, it might be blocking the IR sensor. You can get a little IR repeater kit to solve that, or just rely on the TCL mobile app as a temporary remote.