Is the drain properly vented? Does the drain follow a circuitous route with multiple elbows? What size is the drain pipe from the wall to the plumbing stack?

I think hair slows my shower and lavatory sink. i use a plunger when the flow slows.

I used to use some of the liquids, but I am now on septic, so I avoid the liquid drain cleaners.

Here is a photo of the underside of the kitchen sink:

The two black pipes go down through the floor and travel more-or-less horizontally between the floor joists about 25 feet to near the basement bathroom ceiling where one is a vent and the other goes down into the drain system for the house. I assume the left-hand one is the vent, but there are no vent pipes through the roof.

Solved the problem: It was gunk between the trap and the sink which I didn't notice nor remove when I cleaned out the trap!

That gunk was the first thing I saw but you found it before I had a chance to point it out.