Just watch the vid on the link they explain it all. There is a bunch of new shit. It's obviously nothing ground breaking, why would it be? It was already nearly perfect as it is. They added custom menus (awesome!!!), WU merged (great if you have multiple blogs!), theme customizations from the UI (e.g. header image change, background change, color scheme adjustments). I know the last one was implemented by some themes, but now it's part of WP core, which is great, because now it standardizes the UI. Rather than each theme author having to create their own UI, it's now all built in. New hooks. I feel like I am forgetting something.
As for performance, this is the key now. If you have multiple blogs on one server, then you can run them off of one source. You can cache compliled PHP with APC for example, and now you don't need to cache for each of your installations/instances, as it's all running from the same code. You can pretty much cache entire WP install in probably under 128 MB. Meaning that it's compiled code sitting in memory and ready to respond. Compilation of the source code usually takes the longest. Also combine that with caching like Super Cache, offload your static files to a CDN (e.g. SimpleCDN) and you are golden.