FileVault: What exactly is the optimizing step about? After enabling FileVault on El Capitan the partition is encrypted in a first pass. After this phase is complete a second optimizing phase is starting which takes considerably longer than the first encryption phase. The optimizing step was not present on the first appearance of FileVault 2 (OS X Lion). The encryption phase took the same amount of time back then. So what is this optimizing all about ?

My system is a mid 2014 MBP with 1 TB SSD. 1st phase was done in about 12 hours. 2nd optimizing phase is now at 20 hours and counting.
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 .