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tee() splits a stream into two branches. It seems straightforward, but the implementation requires buffering: if one branch is read faster than the other, the data must be held somewhere until the slower branch catches up.
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The problem compounds in pipelines. Each TransformStream adds another layer of promise machinery between source and sink. The spec doesn't define synchronous fast paths, so even when data is available immediately, the promise machinery still runs.
I don’t remember when 3.9 was, but I’m pretty sure it was after the steering council took over.