Slow drains aren’t good news. Below are some frequent causes
Dirty Pop-Up Stopper-check for hair and soap scum
Clogged Trap-this is the curved pipe under the drain in which debris can accumulate. Place a bucket underneath the trap, remove and clean it. We may be getting into professional plumber territory here!
Drain Pipe Clogs-the drain pipe is beyond he trap and you will need a snake or probably a plumber as access may be difficult.
Plugged Vent-If these measures don’t eliminate the clog you may have an issue with the vent which protrudes from the roof. The vent equalizes air pressure so water can flow freely down the drain. Plugged vents can also cause a sewer smell.