When my bathtub or shower is used the water takes forever to drain out. I have tried using liquid plumber and it helped only slightly. What else can I do?

I'm going to assume that you're talking about a tub/shower and not two separate fixtures. Forget harsh chemicals, to be sure you're going to hafta snake the drain line. First you must have a hand held snake,(see image).
Now unscrew the two bolts securing the chrome tub waste plate and pull the stopper assembly up and out. Now runn your snake down the open hole, (see image) you will run into the return bend of the trap about 18 inches down but once past that you'll only have to put out 10 feet more. Flush the line with a tub full of hot water. Good luck, Tom