For the second, well, I wouldn't be surprised if someone who worked at optimizing Arc could make it run twice or maybe even ten times as fast, while still implementing the full specification. So if a particular language feature is actually useful, and happens to cause a 10% slowdown, I mostly don't worry about it myself... I suspect greater gains would be found from a systematic optimization approach.
Besides, most of the programs I write already run fast enough, so I wouldn't mind a 10% slowdown for a feature that was actually useful. Which, unless I'm presented with new information that changes my mind, I agree with you that Arc's implicit locking is not.
