Ugh. There’s nothing like standing up to your ankles in water and scummy shampoo, because your bathtub is draining so slowly.
All kinds of things, like hair, bits of soap, etc get caught in the bathtub drain, and that stops it up and makes your drain move slowly.
You could use a product like DrainO, but that stuff is chemicals, and bad for the environment. Here’s a tutorial on how to make your bathtub drain properly again, using vinegar and baking soda (which, BTW, is way cheaper than buying DrainO!).