DIAGNOSIS A software glitch or a static charge buildup on the capacitive touch screen. It is not a hardware failure.
FIX Perform a hard reset. Pull the thermostat off its wall plate to disconnect it from power. Wait 30 seconds. Reattach it to the plate. The device will reboot. This clears the temporary memory and discharges the screen.
IF THAT DIDN'T WORK Check for a plastic film or screen protector you may have left on. Clean the screen with a dry, soft microfiber cloth. Do not use liquid cleaners or press hard.
HEADS UP If the lag happens only when the backlight is dim, it's normal. The processor slows down to save power. Wake the screen fully by tapping it before adjusting settings.