/dev/zero doesn't "write" anything in the sense that yes writes, since it's a character device and not a program. The Linux kernel's implementation of /dev/zero does not write one byte at a time.
You're right of course; and actually I believe the kernel will simply provide as many bytes as the read() requested; so the speed should mostly depend on how you access /dev/zero. IE, the user above was using cat and I think with dd and a proper block size it'd be much faster.
I was under the impression that cat automatically used a sane size for reading. Now that I think of it I cannot think of a source, other that to point out my own anecdotal experience.
When I was writing raspbian images to SD cards to use on a raspabery Cat and DD took within a few seconds of each other on an operation longer then a minute. Since then I have been using cat where I could, but I didn't think to right down the numbers though.
Note that cat+gnu awk was faster than just gnu awk - but mawk was faster still (reading a not entirely small file).
And in a similar vein of gp comparing Gnu and Openbsd, note that openbsd cat is a little more convoluted than the simplest possible implementation (at least to my eyes):