No composite video.
Yes there is. Via the 3.5mm jack. Like the pi does.
What I found that it was missing was lag. No lag at all.
Really? Because Wikipedia and the Asus Tinker board home page says line out and mic in from the 3.5mm jack. You positive?
I remember seeing something in the manual about it. After reviewing your sources (remember wiki stands for What I Know Is), it seems that there is no composite, which is not a deal breaker. So I stand corrected.
It would be completely crap playing Dreamcast and N64 games in composite on a 70" UHD TV. But if that is all you got....
I only have one Pi that uses composite out and that is the original 256mb Model A. That is used for my Stargate cab and TV, so I can see the usefulness of having composite with legacy technology. Everything else is HDMI today. It probably was a decisive factor when designing the tinker board, like unshared USB and 1GB Ethernet with 2gb memory and a better GPU.
It flies, but heats up quickly and intensively, which could be a candidate for quiet mineral oil cooling. It's early days but what you get is a better platform, also tried Android 6.0, and having fun getting Google Play to work at the moment.