HTTPS will slow your web site down, but not for the reasons that others have suggested. It will not "suck your CPU", rather it just increases latency by adding SSL handshakes to the TCP handshakes for each connection. This can cause performance to drop a great deal in situations where many connections are required to load the page (e.g. lots of images etc) especially if you've got HTTP keepalives off.
Having the entire site on HTTPS certainly makes development easier - the whole site on HTTPS means your devs don't need to worry about which bits need to be HTTP and HTTPS, and which pages need to switch from one to another, and composing absolute links to those etc.
Previously I've worked on e-commerce sites which used a mixture and it gets pretty hairy trying to switch from secure / non-secure at the appropriate pages, particularly if your site layout and navigation is complicated and reused from one page to another (which it usually is in most sites)
See paypal.com for an example of someone who chose to just put their entire site HTTPS. But I notice that banks rarely do it.