Why does RAID 1 "Mirroring not provide performance improvements"?
Solution 1:
My guess would be that Apple prefers to have one drive handle only write processes as a backup and read/write from whichever one it considers the main drive. As to why this is, I have absolutely no clue. Seems kind of like a big oversight on their part. I've always kind of gotten the feeling that software RAID is pretty low on Apple's priority list.
Solution 2:
These are what we call implementation details.
You can implement RAID 1 in a variety of ways, Apple choose to implement it in a way without any speed gains.