I just erased a newly purchased external USB disk using the Disk Utility program and set it to use the file system APFS Encrypted. I entered a strong password and the whole process took about a minute, then the device was supposedly encrypted and ready for use.
I noted that APFS encryption uses AES-128 which seems good, but the fact that the formatting only took a minute seems weird. Using encryption software like TrueCrypt, to safely prepare a new device takes hours, even days, just to fill the disk with random data before any valuable data is saved onto the device. For this reason, I am concerned that my APFS encrypted device is not secure enough.
Are my concerns legitimate or have I misunderstood something here?