Basically on a PCIe 3.0 x4 maximum performance is 4000GB/s, in practice this is 3200-3500MB/s. With 4k random write, you will get maximum 800.000 to 875.000 iops per second. Consumer-grade SSD's barely manage to write 500-1000 iops when sending the ATA_CMD_FLUSH command unless they are lying which is really bad.

Good read about consumer SSD performance: http://www.sebastien-han.fr/blog/2014/10/10/ceph-how-to-test...