A slow draining bathtub is almost always caused by hair and soap scum buildup in the drain. This fix takes five minutes and no chemicals.
Unscrew or lift the drain stopper. Insert the drain hair tool a few inches and twist slightly. Pull it back out slowly — the barbs grab hair. Repeat until nothing comes out.
Pour one cup of baking soda directly into the drain followed immediately by one cup of white vinegar. Let it fizz for 10 minutes.
Run the hottest water for 60 seconds to flush loosened buildup. Reinstall the stopper and run the tub. The water should drain quickly and evenly.
Install a drain hair catcher over the bathtub drain. Empty it after every shower and you'll rarely deal with a slow bathtub drain again.