[jazzypants]

Userland implementations are never as performant as native implementations. That's the whole point of trying to add immutability to the standard.

[agos]

even when performance might not be an issue or an objective, there are other concerns about an user land implementation: lack of syntax is a bummer, and lack of support in the ecosystem is the other giant one - for example, can I use this as props for a React component?