We have a double vanity in our bathroom which was converted from a single. It doesn't drain well at all, sometimes takes 24+ hours to drain. The toilet and tub drain fine. I've disassembled the drains under the sinks and found no clogs. Ran gel clog remover through it, no change. Used a bladder and hot water which seemed to go through just fine, water was flowing at the exterior cleanout. I'm thinking at this point maybe a venting issue? There's not an under sink drain do I thought of adding one but wanted to see if I'm wasting time and money. Plumbing is nowhere near being my specialty. A photo of the plumbing is attached.