I've run several secure erase operations on my 500GB Samsung SSD 850 EVO using the procedure described on the Linux ATA wiki and it takes only a few seconds to complete:
linux-203v:~ # time hdparm --user-master u --security-erase foo /dev/sdb security_password="foo" /dev/sdb: Issuing SECURITY_ERASE command, password="foo", user=user real 0m2.745s user 0m0.001s sys 0m0.003s
When viewed in a hex editor after the erase, the drive is indeed completely blank.
Is this normal? Has the drive actually erased all NAND cells this quickly?