Hair binds with soap and shower gel residue to form a dense fibrous plug that water passes through slowly. It builds over months, so the slowdown is gradual — which is why people tolerate it far longer than they should.
Shower trays with a hidden trap under the tray need the waste grille unscrewing. If access is impossible, the run can be cleared from an inspection point or with a mini-jetter.
If several bathroom fixtures are slow at once, the problem is not hair in one trap — it is the branch or stack they share, and needs professional clearing.