When you overwrite the disk, you say you should use dd if=/dev/random, but the reason why this is much slower is because /dev/random blocks. Use /dev/urandom because it doesn't block -- Before you ask, yes this is still secure.
Or you can use dm-crypt and create a temporary container with a random key to wipe it. This will be a bit faster than the above. See the Arch wiki for details[1]
On the subject, is there any evidence that wiping to anything more complicated than 0s is actually helpful?
Are there are practical attacks where you take a zero'd hard drive and derive data from it that you wouldn't have been able to derive were it written over randomly?
Or, you can use the (S)ATA Secure Erase[1] command which was designed from the start to securely wipe a hard disk and should handle wiping reserve space etc. properly.
Apparently, this is not always implemented correctly on SSDs. It should take quite some time, but if it finishes after a few seconds, it probably didn't really wipe your SSD securely.
Most SSDs should be encrypted by default and all secure erase would do is to wipe the encryption key and generate a new one which takes no time and does effectively and securely wipe the entire disk.
Or you can use dm-crypt and create a temporary container with a random key to wipe it. This will be a bit faster than the above. See the Arch wiki for details[1]
[1] - https://wiki.archlinux.org/index.php/Dm-crypt/Drive_preparat...