The patch adds a non-iov_iter version of reads for the /dev/zero interface. The write performance is unchanged as it already had a non-iov_iter implementation.
The patch by prominent upstream kernel developer Christoph Hellwig explains that in fact there is a "major speedup" and it's attributed to the iov_iter overhead in the read_iter path, " Christophe reported a major speedup due to avoiding the iov_iter overhead, so just add this trivial function. Note that /dev/zero already implements both an iter and non-iter writes so this just makes it more symmetric. "
As for how the overhead can be so bad with something as simple as /dev/zero , any further details are pending on the kernel mailing list. In any case, the patch is now in char-misc-next ahead of the Linux 5.10 merge window opening next month.