I interpreted that statement as "this is available in API level 19 and up", not "this was introduced on the same date that we originally released API level 19", but it looks like you're correct: https://www.youtube.com/watch?v=zxHVeXbK1P4 That's way older than I was expecting.

Admittedly, there were bugs on the initial versions and wasn't really widely useable until about API 23 or so. But that's still years back.

I think certain performance issues have persisted until today, though?

If you need to do stuff like recursively enumerate files in a shared directory, it's slow without workarounds to avoid the extra IPC calls. For stuff like syncing the shared directory in the background, the performance impact is not material (20-30ms per file).