LPT: Before calling a plumber, check if your slow drain is just a buildup of hair and soap at the removable stopper. Takes 2 minutes and costs nothing.
I say this as someone who paid $180 for a plumber to pull out a clump of hair from my bathroom sink last year. He was there for eleven minutes. He was very professional about it and did not make me feel stupid but I thought about it for weeks afterward.
Most bathroom sinks have a stopper that you can either unscrew by hand or pop out by reaching under the sink and unscrewing a small pivot rod. Once it's out you'll probably find a matted ring of hair and soap residue sitting right at the top of the drain that looks like it's been there since the Obama administration. Pull it out with your fingers or a bent paperclip, rinse the stopper, put it back. Drain fixed.
Same goes for shower drains. Most of them have a cover that lifts off or unscrews with a flathead. Whatever is under there will be unpleasant to look at but it is not a plumbing problem. It is just hair and time.
The actual rule I follow now: if a drain was working fine and then gradually got slower over a few weeks or months, it's almost always a buildup issue and almost never a pipe issue. Clean it first. If it's still slow after that, then call someone. But in my experience and the experience of everyone I've mentioned this to, the slow drain fix takes under five minutes and costs nothing and makes you feel unreasonably competent for the rest of the day.
The $180 was a good lesson. Slightly expensive but I have not made that mistake again.