clean up byte buffer copy #3560

Job	Run time
macOS / release / homebrew-clang	6m 58s
macOS / debug / homebrew-clang	4m 56s
macOS / release / system-clang	4m 23s
macOS / debug / system-clang	3m 52s
Windows / release / msvc	17m 25s
Windows / release / clang-cl	15m 7s
Windows / release / llvm	19m 2s
Windows / debug / msvc	23m 55s
Windows / debug / clang-cl	22m 14s
Windows / debug / llvm	25m 51s
	2h 23m 43s

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