> Errors as return values is only acceptable for code that is so performance-sensitive that you aren't allowed to do dynamic memory allocations.

Exceptions-based code can be zero-cost, if no errors occur, at an increased error-case cost. Using error values pessimises this, and increase branch prediction load (as every callsite is now a branch).

So in the common case where errors are extremely rare, exceptions-based error handling can be quite a bit faster than return-value error handling.

Which doesn't mean it's preferable, but beware thinking that return values are faster.