Hair and soap, almost every time. The exception is when several bathroom fixtures go slow together — that is a different problem and worth treating as one.
Bathroom drains are the simplest blockages in the house and the easiest to prevent. Hair and soap bind into a mat at the pop-up or the strainer, water slows, and eventually it stops.
What we pay attention to is whether it is only that fixture. One slow shower is a shower problem. A slow shower, a slow basin and a toilet that empties lazily is a branch or main-line problem wearing a disguise.
Blocked drains rarely travel alone. Here is what else we handle.
Cleared quickly, and checked for whether it is only that fixture.