Does FileVault encryption slow read/write access on an SSD?
Every time you make the computer do something extra, in this case encrypting/decrypting all file access, it will take longer and the machine will slow down a bit.

FileVault 1 did slow things down noticeably, but with FileVault version 2 (Introduced in OS X 10.7 (Lion)) running on an SSD there is no noticeable decrease in reading or writing files. I have this enabled on a 15" MacBook Pro Retina and a 2012 11" MacBook Air. The only time it is noticeable is when you reboot, as it requires a password before starting the boot process.

You may notice that the system is slow when you first enable FileVault 2, since it has to encrypt the whole drive. Once that is done you will probably forget that it is on.

If you decide that you don't agree with me, turning FileVault off is easy enough. You will, once again, have the slow initial period as the whole drive is decrypted.