Make note of performance implications of Read

[steveklabnik]

Fixes #28073

[rust-highfive]

r? @nikomatsakis

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[nikomatsakis]

@bors r+ rollup

though I might add a nit to say something like "will likely be faster" :)

[bors]

📌 Commit 367f46d has been approved by nikomatsakis

[steveklabnik]

I thought about it, but I can't imagine an instance where not amortizing syscalls ends up being slower... at the least, should be equal. :)