Plumbing issue...perhaps venting problem
That looks like a DIY or (cheap) trap. As it will work the same even if the ends are swapped , I might look at the sink itself. 

Not long ago I found a sink that had clear drains and a good vent but was draining slowly. 

I filled the sink with hot water until it started to slow then used a toilet plunger , what pumped out of the sink vent was so gross, i don’t want to say what it looked like but after a couple of plunges it started draining better. I had cleared the trap and drain with a small power auger, it was only after plunging that the in sink vent cleared and it drained well, after it started draining I filled it to the sink vent level and let it run on hot for a while to make sure it it was clear. 
This is in a rental so I don’t know what was plugging the sink vent and it was my first experience like this but a plunger with hot water may help you as it did me. Once the vent started to clear I plugged it with my finger while plunging prior to over flowing to the vent for everything I did. Hope this helps.