When I was wearing a heart rate monitor, my pulse dropped by a large amount when I set on the toilet.
It was the action that I could do with produced the largest drop in heart rate. The effect was much stronger than lying down, starring at the wall and doing nothing.
Evolution seems to have built a relaxation program that automatically starts when one sits on the toilet. To complete your task in the bathroom in the shortest amount of time, you have to relax the muscles in your anus.
If that relaxation program gets disrupted you will need more time in the bathroom. It therefore seems ineffienct to do other tasks while you are in the bathroom.
If you want the process to be more efficient, do progressive muscle relaxation. Use the time on the toilet to relax as deep as possible.