The slipper was the most common punishment at my school, and not that common and done in private so often we did not even know a boy had been punished. I can recall two occasions where I knew a boy had been slippered. Both times the boy was a bit redfaced and possibly had been crying.
On the other hand an older boy (11 ish) who was brought back into the classroom after being caned and did not show any signs of being punished, no red face, no sitting uncomfortably.
Another boy who was a friend of mine was one of the more regular recipients of the slipper and cane and he said that he thought the headmaster (an older large man) was not particularly effective with the cane! Perhaps that was bravado on his part, I did not try and find out for myself.