What is your source? I'm wondering why...
I don't agree with his statement, however it isn't unfounded.
For example:
The Intel 520 SSD uses compression to increase its overall speed. Now if the data on the drive is encrypted by software such as Bitlocker or Truecrypt the controller won't be able to compress the data all that well because encrypted data isn't very suitable for compression.
(The Intel 520 uses NAND that is already encrypted with AES 128, so using an ATA password in BIOS will give you the bonus of FDE without the performance hit.)
However, an SSD running on Bitlocker is still worlds ahead of a mechanical disk that isn't encrypted at all.
I encrypt all my drives (Which happen to be the Intel 520). My laptop has Secure ATA password support, so i use the inbuilt encryption there (No performance hit), but my desktop doesn't support this feature so i encrypt it with Bitlocker. There is a performance hit on the desktop due to the software encryption, but for a workstation which doesn't demand extremely fast and high throughput I/O the system still feels as fast as before it was encrypted.
The 520 has a read/write of 550/520 so if we (for arguments sake) say that after being encrypted that drops to 400/250 then you still have an extremely fast disk with an access speed which destroys mechanical disks. So unless you have extremely high I/O requirements for your system then there shouldn't be any reason to not use FDE. If you can leverage the inbuild encrypted NAND then you can have FDE without any penalty at all.
I might also mention the Crucual M500. I support eDrive technology where Bitlocker can leverage the onboard encrypted NAND which again gived the bonus of FDE, but with an extremely small performance hit (This has been covered on an article here at Anandtech).
While i've been typing this i'm running an ATTO benchmark on my Intel 520 which is encrypted with Bitlocker so i'll post the result when it's done.
Edit:
Bitlocker encrypted Intel 520 due to lack of ATA password support on Desktop
Writes take around 50 to 55% hit and reads are pretty much untouched. Again, depending on your requirements this may or may not be a problem. But, again, leveraging the inbuilt NAND encryption will yield perfect performance (100% performance)