I have a slow draining sink. I think: "No problem. Just perhaps some black slime in the trap" [I've had that before in a different sink]

The sink drains but it is very slow. The other things in the bathroom (toilet, shower) seem to flush/drain just fine.

So I put a bucket under the trap and loosened the slip nuts. To my surprise more water drained into the bucket than could be in the trap. Apparently there is standing water in the pipes in the wall. I tightened up the slip nuts and stopped when I wasn't sure how much water is standing in the pipes.

The sink drain goes horizontally into the wall after the trap.

So do you think the pipe in the wall doesn't have enough slope? I suspect to goes about 4 ft to the left to hook into the bathroom stack & vent that I can see in the attic.

I do have an attic-installed HVAC geothermal unit that I drain down into the vent. If the vent was clogged then the whole bathroom wouldn't drain right so I don't think the vent is the issue.

Did I miss anything in my analysis?