FileVault: What exactly is the optimizing step about?
FileVault 2 is optimizing data on your system drive before it gets full disk encryption (FDE) to speed up encryption/decryption process. Sometimes optimization stage can take a very long time to complete. It depends on a capacity of your disk and an amount of data it stores on.

You can use coreStorage command in Terminal to see the status of encryption:

diskutil cs list


or:

diskutil cs list | grep -e "Conversion" -e "Volume Name"


To see the other useful commands look at this page: Set a FileVault recovery key for Mac .