I never look at my speedo. And in any case it'd depend on the radius of the switchback. Some of them are really tight and require lower speed, others aren't so tight and you can go faster.

The problem with the early apex is that you have to slow down on the exit. When I'm descending with a group I often lose a bike length or two on the corner entrance but I exit with more speed and am right back on the wheel.
Late apexing is often a good way to pass.

For pre-riding I just aim to memorize the road. Just knowing "this corner opens up on the exit, the next one has a nasty bump on the inside line so stay outside" helps my confidence, which helps speed.