No increased performance when adding more vdev to mirror

Samsung 850 EVO (512GB) x4 striped

root@freenas:/mnt/benchmark # dd if=/dev/zero of=tmp.dat bs=2048k count=50k
51200+0 records in
51200+0 records out
107374182400 bytes transferred in 79.603552 secs (1348861702 bytes/sec)

root@freenas:/mnt/benchmark # dd if=tmp.dat of=/dev/null bs=2048k count=50k
51200+0 records in
51200+0 records out
107374182400 bytes transferred in 67.587923 secs (1588659307...