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 or not fully clearing the sink or tub.
Some potential solutions include using a plunger or drain snake to try to clear the blockage, or using a chemical drain cleaner. However, if the clog is severe or the issue persists, it's best to call a professional plumber to diagnose and fix the problem properly.