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> borrow is never tasked with cleaning up data

And the reason for this is that there is no branch needed at the end of the borrow, to check for refCount=0? (Or, is this at least one of the reasons?) I'm wondering about the performance impact for this... There's also the code size impact.

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Not quite. A borrow doesn't do cleanup because it's, well, a borrow, i.e. a temporary reference to some owned value. It's the owned value that's tasked with disposing of any resources/memory when it goes out of scope, just as in Rust.

The cost of borrowing (at least with heap types) is an increment upon creating the borrow, and a decrement upon disposing of the borrow. Over time I'd like to optimize those away, but that's not implemented at this time.

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