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.