Technically, yes. There is a performance cost. Personally I've never been able to notice the difference. From a UX point-of-view, not really, I guess is the answer.
If you are in a field where encryption and protection of data and connection are important and/or necessary, you have to take the hit out of necessity and just work with it. But nothing I've done so far has shown any noticeable slowdown.
Technically there are supposed to be a penalty of approx. 5% with SSL enabled.
In real world experience, I have not been able to notice the penalty.
I can only second Claus.
We have not seen any significant or measurable slow down due to SSL (encryption).
If you are facing performance problems you can of course, as an experiment, try to turn of SSL and test with/without.
And since it is primarily the processor that is drawn on when encrypting/decrypting it is worth mentioning that on most FileMaker Servers the processor is not overloaded.
In the future with more intense use of WebDirect of on heavy load servers with real many users or very complex solutions 5% may be of importance. But in most cases we have seen it is not.
I believe that SSL and encryption will be the standard that we have to implement for all servers unless some special reasons tells us not to.
We run FMS 14 with a 100% Webdirect solution.
We notice no significant loss of performance at all due to SSL.