I just bought my first house and all my drains tub, 2 toilets, a shower, and my washer downstairs in my basement takes a while to drain. The house was built in 1925.

Unless your house has been repiped you have 1 1/2" galvanized drain lines going into a 4" cast iron main. These pipes can build up crud on the pipe walls and have to be snaked and flushed every once in a while. Perhaps it's time to call in outside help and snake and clear your drainage system. Good luck, Tom