It is well known that ZFS is built for Enteprise settings, with many disks and large raids on heavy servers. It it built to scale. For instance, IBM Sequioa supercomputer uses Lustre on top ZFS, and stores 55 PB disk, and achieves 1TB/sec bandwidth. ZFS does not perform well on single disk settings. It is not really a desktop filesystem.
Ted Tso, ext4 creator, explained that ext4 is best with few disks, i.e. it is designed for desktops. Not large servers. Ted Tso, himself, does not have access to large servers with 100s of disks, so how could he optimize ext4 to large storage servers? He has a fast desktop PC, and some disks and can test and optimize and debug for that scenario. If he was given large servers with 100s of disks, then he could tailor ext4 to those servers.
However, it would be interesting to see how ZFS L2ARC and ZIL changes everything. To use SSDs as cache is an Enterprise feature and really boosts ZFS performance. For ZIL you should use a very low latency SSD, preferably ZeusRAM which is battery backed up RAM. And for L2ARC, use SSD with low latency. This setup can boost performance into 100.000s of IOPS!!! But, ZFS shines with Enterprise hardware, SSDs, large RAM, etc. Nothing can touch ZFS in that case.