The answer to 'why is my pipe slow draining' is that a clogged or partially blocked pipe is the most common cause. This can happen due to buildup of hair, soap scum, food particles, or other debris over time. Symptoms include water draining slowly, gurgling sounds, or standing water in the sink or tub.
Some simple DIY solutions to try first include using a plunger to dislodge the clog, or pouring a baking soda and vinegar solution down the drain followed by hot water. However, if the clog is deeper in the pipes or the problem persists, it's best to call a professional plumber to use a drain snake or other tools to fully clear the blockage.