Actually, in this article, Linux writing to RAM is slower than MacOS writing to SSD. Hard to conclude that "it's just OS X being slow". I suspect the hardware used is wildly different from the one in the original article.

'In the above example it can be seen that an input block size of 128K is optimal for my particular setup.'

However, the SSD seems to be significantly slower, so the relative overhead of the syscall is going to be respectively less:

'Reading from /dev/zero and writing out to a SSD with a more optimal block size of 256K yields a throughput of 280 MB/s.'

I was getting 250MB/s with the 512 byte buffer size, although the test in the article is doing two syscalls (one read(), one write()) whereas mkfile is just doing one.

At 256KB buffer size I was already getting around 1.8GB/second writing to SSD.