Does passphrase length impact read-write speed? I'm using Diskcryptor to encrypt a 2TB external USB HD and the process is taking over 2 days. I chose a 100-character strong encryption key passphrase (because, why not?) and am now wondering if the length of the key passphrase has any impact on how long it takes to encrypt the drive. I'm also wondering if the length of the key passphrase will have an impact on how long it takes to write and read files from the encrypted drive. If I had chosen a 25-character key passphrase would this process be faster? Would reading and writing from the drive be faster?

Update: It has been explained to be below that what I was calling the encryption key is in fact the passphrase.
If I had chosen a 25-character key would this process be faster?


That would depend entirely on how Diskcryptor was designed; but it appears it would not.

According to the Diskcryptor homepage, Diskcryptor selects a random symmetric (AES, Twofish or Serpent) key, and uses that to encrypt the disk; it uses the passphrase (what you call the strong encryption key) to encrypt that random symmetric key and put it in the boot sector.  When it comes time to use the disk, you'd enter your passphrase, which would allow Diskcryptor to decrypt the random symmetric key; that symmetric key is then used to decrypt (and encrypt) everthing else.

Because the vast bulk of the work is done using that random symmetric key (which is the same size independent of the size of the passphrase), the length of the passphrase doesn't matter.

Now, the page does mention that it can use Cascaded ciphers; say, AES + Serpent.  If you have it configured that way, it would be considerably slower than if you were just using AES (which would be easily the fastest because of the AES-NI instructions); that's the only thing I can think of which might be slowing you down.