DIAGNOSIS The SolarCell remote uses RF (radio frequency), not IR, so it shouldn't need a direct line of sight. The lag usually means it's low on power or there's interference from other wireless devices.
FIX First, give it a full charge. Even though it charges from ambient light, it might have shipped with a dead battery. Place it under a bright LED lamp (not in direct hot sunlight) for a few hours. The red light on the back should come on while charging. After charging, press and hold the back button (next to the volume buttons) and the Play/Pause button together for 5 seconds to re-pair it with the TV.
IF THAT DIDN'T WORK Check for sources of RF interference. Other wireless devices like routers, wireless speakers, or even USB 3.0 cables near the TV can disrupt the signal. Try moving your router a few feet away from the TV. Also, make sure there aren't any large metal objects between you and the TV.
HEADS UP If the remote is still unresponsive after charging and re-pairing, the internal capacitor might be faulty. Samsung will often replace these under warranty if you contact support, as it's a known issue with some batches of the SolarCell remotes.