My Shower/tub doesn't drain well

And it is not clogged, oddly enough, that I can tell. I have used Drano, etc and nothing works for long. It appears to be the type of unit that simply doesn't drain fast, or well at all. I have to clean up hair every few days and even after I do weekly cleanings, it still leaves residue, dirt, etc in the bottom. It's one of those that has the part that goes up and down by twisting it to the left or right. However I cannot "unscrew" it to remove the part altogether. Something has to be done, it won't flood or anything, but is is huge inconvenience, and has potential to start leaving odors.

I have the same problem in one bathroom. I suspect the pot-smoking plumbers that installed the original plumbing in my house broke some basic rules of plumbing.
I am not a plumber, but I'm guessing they didn't angle some pipes correctly, provide adequate venting, or used the wrong size pipe in the system.
I will call in a plumber someday, I am living with the slow drain for 7 years now, I realize that if a good plumber can solve the problem it may require removal of walls or ceilings. It may not be worth the hassle to fix the slow drain.

I bet you've got one of those lift & turn stoppers... I hate those things...always catching trash and hair and clogging up the drain. If so, lift it up and get your head down to where you can see the side under the lip really good... have a flashlight or good lighting. There's a flat piece that stick out to the side of the center piece. Look for a hole with a slot screw head down in it. It's like a set screw. Back out that set screw and pull the stopper out. There will be a post in the drain. If you want to do away with the original stopper you need to screw this post out (might have to use vise grips). Otherwise you can just clean out the crap under this stopper and reassemble.

Why not clean the darn drain out the way it should be cleaned . With a proper snake. Not a magic potion.