The same very old, mostly misconstrued theory still holds for the S8: Encryption slows things down. It's just how it is since you're adding a layer of security that has to be decrypted for it to be used, and that takes more processing than a non-encrypted one. Will that, however, be noticeable? That depends on the device, and on most modern devices, it's negligible.
On top of that, if you encrypt the SD Card but not the internal memory, since the S8 (AFAIK) doesn't support Adoptable Storage, will not be as affected since it's only decrypting data off of the removable storage media and not its 'everyday' memory.