How many web servers do you know of which are CPU bound, and not through massive code stupidity, and not I/O (in some manner - waiting on SQL, disk access, bandwidth)? Encryption can run while other threads are waiting for a response.
In general, it's a negligible cost; it adds a very minor delay compared to latency / transfer time, and uses CPU otherwise highly unlikely to be pegged. If you're pushing threading limits / CPU usage limits, you're probably inches from needing new hardware anyway, and SSL should be considered part of the cost of running a web server.
