Will enabling FileVault convert my HFS+ disk to APFS? I have a 2017 5K iMac running Mojave 10.14.6.  It has a 2 TB Fusion drive internally formatted as HFS+.  FileVault is not enabled, but I wish to do so.  Will the disk have its format changed to APFS if I enable FileVault?
My recollection is that the disk was originally formatted as HFS+ from the factory and when I turned on FileVault, the format changed to APFS.  Granted that was over a year ago, so I could be very off-base.
I want to prevent going to APFS again.  The performance degraded to dismal levels.  Now that I have reformatted as HFS+, it is surprisingly speedy.
FileVault and APFS are two very different things; enabling one doesn’t enable (or necessitate) enabling the other.
FileVault is encryption - it scrambles, to put it simply - the contents of your drive with a known key.  Specifically, it uses a 128bit AES encryption with a 256bit key.  See Use FileVault to encrypt the startup Disk for additional details.  You can encrypt JHFS+ formatted volumes as well as APFS volumes.
APFS (Apple File System) is simply Apple’s next generation file system.  As far as encryption goes, it supports it natively unlike JHFS+.