A rule of thumb: SSL will make your site about 3x slower.
More or less.
That's a stat that a sysadmin once told me. I'm inclined to say that's a conservative estimate.
As the server is working 3x harder to decrypt all the requests, consequently your server will be able to handle 33% of its usual load.
Watch your CPU usage when SSL is turned on. Maybe you just need to throw more hardware at the problem.