I’ve had my house for 10 years and my upstairs tub is pooling up after 5 seconds of running the faucet. It does have a slow drain. I’ve snaked the drain to about 5 feet until I bent the snake and the next one I couldn’t get from stopping coming into the bottom drain and not where I needed it to go. The upstairs sink and toilet is not affected and neither is the sinks on the main floor and also neither the sink/toilet or old fashion sink that the washing machine drains are affected …only the upstairs bathroom. It doesn’t have that drain lock so there’s no linkage stuck problem I’ve been seeing. I just use a plug to stop the water. I’ve poured some lye down it one day and some liquid plumber which I know isn’t good. I just don’t want to end up ruining my pipes——any solutions. My pipes I believe are pvc

as long as it is draining a little bit then you can try some liquid drain cleaner.
just be careful with the stuff.
make sure all the water has drained out of the tub and pour some into the drain. leave it for a few hours and dump a small bucket of hot water from the sink into the drain.
you most likely have some hair clogging the pipe.

please be careful.

also... there are lots of videos on snaking a tub drain.

don't snake if there are chemicals present.

One must FIRST remove the trip lever assembly (if any).

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638