Sanchin breathing is not quick it follows the movement of the arm. The tension performs an strengthen function of the ligaments, tendons and muscles of the entire body, it especially strengthens posture and leg muscles even the feet & toes if done properly.
The sequences are slow so as to concentrate on total body and corodinate movement with breathing. Usually in the intermediate stage the Kata performer is struck and slapped (on the forearm, shoulders, backside), kicked between the legs, thighs, stomach, boards broken on your fist, head, thighs, forearms, all this is ignored as you contiune rythmatically movement and breathing. You are eventually able to absorb punches/strikes to pressure points like solar plex, even repell the attacker away as if he has ran into a wall. This is developed from basic to shodan/intermediate level then you really start training. It is the basic of Gung-fu's Iron body training it is not a dance or just to get a trophy, its about vitally and health. It opens the door between your external/hard and Internal/soft training.
It is said that Hagashionna Master could stand in sanchin with two people on opposite sides, pulling on an obi looped around his neck he could not be choked out.