Bathroom Sink Drain slows down

I have to remove the trap on my bathroom sink to remove a build up of black Jello-like stuff twice a year. It only seems to build up significantly on the metal pipe attached to the sink itself. I use liquid hand soap. I'm the only one who uses the sink. What is causing this?
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around a clump of hair, which gets caught on the lever thingie for the stopper. I <removed> the stopper in my apartment sink, to speed up the drainage, which seems to help the problem. Did it on tub, too which had same problem.
Changing the trap to a fresh plastic one with nice slick innards may help. Make sure to deburr any cut pipe ends. Any little snag will build up stuff. Prophylactic use of Drano or something once a month, as nasty as that stuff is, can also help. If you don't mind flicking black crud around bathroom as step one of the cleaning process, one of those long skinny bottle brushes (think giant pipe cleaner) shoved down drain weekly can also help. Just wrap a rag around it as you pull it back out.
My experience was the opposite of yours- I had more problems with hand soap than liquid.
aem sends....
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The pipe coming off the sink is degrading. In fact, it is jagged at the end! I guess I should replace it. This as you suggest probably would provide a smoother surface. The trap is the modern plastic type.

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