HTTPS creates more overhead. That's fine if you have a fast PC but it can really slow down web site servers if they are not designed to handle it.
I've written software that has then been deployed to web farms at big companies, and in my experience the CPU use by the web servers went up by ~30% (average CPU use over the period of a week) when we turned up HTTPS. This was on a web application that had ~1000 concurrent user sessions.
When I was applying load test tools to see where the user experience of my different web apps starts to degrade, what I've found is that when you use reasonably high end hardware for both your web servers and your database servers, your database server will limit the scalability of your web app long before your web servers do, and this is with a single server dedicated to each task. I've done this test with many custom and commercial off-the-shelf web apps/products, and the results are always the same. Relational databases are where the bottleneck is.
Not to mention that it is very easy to scale out on your web tier just by adding more web servers behind a TCP load balancer provided the web app in question doesn't do anything stupid with server-side session variables, whereas scaling up a relational database is substantially more complex.
So, don't worry about the additional CPU load caused by SSL/TLS on web servers, it's not that big of an increase, it scales well across CPU cores, and can scale easily to multiple web servers. Turn it on, or people will be able to hijack your cookies and steal your session using tools like
FireSheep.