Although BitLocker encryption occurs in the background while a user continues to work with the system remaining usable, encryption times vary depending on the type of drive that is being encrypted, the size of the drive, and the speed of the drive. If encrypting large drives, encryption may want to be scheduled during times when the drive isn't being used.
When BitLocker is enabled, BitLocker can also be set to encrypt the entire drive or just the used space on the drive. On a new hard drive, encrypting just the used spaced can be considerably faster than encrypting the entire drive. When this encryption option is selected, BitLocker automatically encrypts data as it is saved, ensuring that no data is stored unencrypted.