Usually a mat of hair and soap scum at the trap — the most common cause by a wide margin. After that: conditioner and product build-up coating the branch, a roughened cast-iron branch in an older home catching hair faster than it should, or — if other fixtures are involved — a main line problem showing up at the lowest fixture.
Camera first — you see the pipe before any price is quoted.
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The hair-and-soap mat: what's actually down there
Hair doesn't clog a drain by itself, and neither does soap. Together they're a building material. Strands catch on the crossbars of the drain fitting and on any roughness at the trap, and soap scum — the curd that soap forms in hard-ish water, helped along by conditioners and body washes — cements the strands into a mat that behaves like felt. Add the biofilm that grows on any permanently damp surface and the mat gets slick, dense and surprisingly strong. A clogged shower drain Nashville call is almost always this mat, sitting within arm's reach of the drain opening, which is why the fix is a short tool and not a machine: a hand auger worked through the trap comes back with the whole grey rope of it, and the shower drains like the day it was tiled.
Tubs play the same game on a longer clock — the tub's trap arm runs further, and a mat that forms past the overflow takes a cable through the overflow opening rather than the drain. Either way it's branch work, single-fixture work, one written price — and the mat itself is the diagnosis. When the cable comes back clean from a slow shower, that's information too, and it points somewhere else entirely.