What are some causes/solutions for poor drainage?
It seems that our house tends to drain water very slowly. The bathroom sink could probably overflow if the water was left on, the bathtub also seems to take a while. Even the kitchen sink seems to drain slowly (though turning on the garbage disposal seems to help).
I've already poured Sulphuric acid down the bathtub, but that hasn't made much of a difference that I could tell.
edit:
We have public sewer
Since all of your drains are slow, you could have blocked or broken connection to the sewer system. There are companies you can hire that will come in and put a camera down the drain pipe so you can see blockages or breaks. It's been quite awhile since I had it done, so I can't remember exactly what it cost, but it seems like it was $200-$300.
My initial thought is that the proper venting is not established is your plumbing system. You need to have "air behind water" to allow everything to drain properly. Essentially it is just a vent pipe that leads to your roof so that pressure does not build up and slow the draining of your sink/tub. Just like how you would punch two holes in the top of a juice can to allow it to poor smoothly... the second hole acts like a vent. Reference: http://en.wikipedia.org/wiki/Drain-waste-vent_system
Try something like Draino or Liquid Plumr - both are alkaline bases, which may dissolve a clog that an acid won't.
For the tub especially, you may want to try just taking off the cover (if there is one) and looking to see if there is hair or something blocking it - it is easiest just to remove the stuff you can manually, then put a drain clog remover down.
If it's a worse clog, you can try using a snake, like this or this one.
I'm going on the assumption that it's not the entire house that's a problem here, but it could be your main line clogged (though you'd also be having problems with toilets, in that case).
I would not try Drano or other harsh chemicals as these are only slightly effective if the clog is close to the drain. When all drains are draining slowly, the first step is to check the main drain and sewer lines. You can usually find a plumber that will include a video camera check and sometimes this is free if you are using them to make the repair.
