It isn't TOO bad - most sites I've looked at, it seems to come down mainly to compression, cache, and optimizing javascript/css/images.
Code on most of my sites (not counting old ones that I plan to rework ... someday ...) was pretty well optimized to begin with. When they first started talking about it I went around and minified all my javascript files and moved as many as possible to the end of the source code. Still need to clean up cacheing on a bunch of my sites. That's one of those things I don't like to implement while I'm still making a lot of changes because it's a nuisance during testing, but then once the site's up and running it tends to get forgotten. Another thing to add to my checklist.
Compression won't be an option for many sites that are on shared hosts - most of them are reluctant to implement it because it takes up a lot of system resources. You can do it if you're on a dedicated or a VPS, but then those platforms are faster to begin with.