Not enough information to verify the usefulness of your benchmark. Were the volumes/partitions aligned? What technology/cipher/keysize did you use for the FDE?

Not enough information to verify the usefulness of your benchmark. Were the volumes/partitions aligned? What technology/cipher/keysize did you use for the FDE? Does your CPU have the AES-NI instructions? Does the encryption tech that you're using take advantage of those instructions? FDE on any type of media causes no slow down if the CPU can encrypt/decrypt at least as fast as the disk can transfer data. It's not correct to say that FDE always causes slow downs.