Copying a bunch of stuff because the borrow checker won't let you share it can drag down performance as well. Yes, I do understand why one might conclude that tradeoff is worth it. But it is a tradeoff.
Funnily enough, because the borrow checker is so strict I feel more confident writing complex borrowing logic that I wouldn't dare attempting in C or C++ because even if I were to get everything right (a big if), there's no assurance that a later refactor wouldn't subtilty break the code. The borrow checker sometimes makes you copy data that you thought you didn't, but more often than not it is enforcing an actual edge case that would have been a bug, had the borrow checker not be present. If the copy is indeed so critical, you can also ease your pain with runtime checks instead using Rc/Arc, but that's another discussion.
