Generally, they are designed to naturally make the driver slow a little to see other cars on approach. Often it's paired with slight turns just before the intersection which also does the same thing partly. Some places (most?) actually put obstacles like trees to make it harder to see other cars from further away.

I suppose it depends on what is meant by traffic and I suppose it does depend on whether "slows traffic" means slowing down compared to just zooming past with a green, or drives at a slow speed. I would think that a) it slows traffic compared with zooming past and b) it's not that slower

On average, I suppose, smoothly moving traffic at slightly slower speeds will be more efficient than 3 lanes of stopped traffic and 1 of fast.