It depends what you mean when you say it buffers. If it’s transcoding then one issue is that the LG TV client doesn’t support the video and it’s trying to convert on the fly. If it is, then it isn’t a bandwidth issue.
It’s debatable whether the WiFi on the Apple TV is faster than the LG TV. But it should direct play nearly all videos, and moreso if you use Infuse player (paid app/subdcription).