Why did my tub drain start running slowly after going unused for a little while?
I'm guessing that the soap scum hardened. It's initially soft and can get partly flushed out with regular use. Now that it's hard, you'll need chemical and/or mechanical means to clear it. You can try the usual home remedies, they might get it flowing adequately for a low flow shower. You might as well invest in a plumbing snake. I'm sure that's all your plumber did. A minimal length manual snake is cheaper than a plumber and it's available 24/7 at no additional cost.

All a snake often does is clear a narrow channel through the gunk. Unless the clog is held together with hair, a good part of it will remain, meaning the drain will eventually clog again. Tub drains are essentially clogs waiting to happen. Unless all the parts are accessible and able to be disassembled, there's little you can do but keep a narrow channel reamed out. To use the snake, remove the lever at the overflow assembly along with the attached parts. Feed the snake down through the overflow and into and through the trap below the floor. The short horizontal bit between the drain and overflow is accessed through the drain itself. You usually need to remove some parts here as well to gain access.