Everybody expects magical perf if you do that when writing files. We have RAM buffers and write caches for a reason, even on fast SSDs. We expect it so much that macOS doesn't flush to disk even when you call fsync() (files get flushed to the disk's write buffer instead).

There's some overhead to calling write() in a loop, but it's certainly not as bad as when a call to write() would actually make the data traverse whatever output stream you call it on.