There are actually two kind of performances, which are both important. The real, actual performance, and how well the OS can make the system feel even under load. It's important to have a snappy feel even if the system underneath is working hard, and this is especially true now that the amount of cores in CPUs and multithreading are increasing. Say what you want, but just the feel of speediness is an important factor. This is why the boot up time is looked at so much too - it's great to quickly get to the desktop, and let the OS load up while you're already started working.
One thing I've noticed with boot up times (and this applies to all operating systems) is that the OS tries to load all programs at once. Usually the limiting factor to this will be hard drive. It's less true with SSD drives, but it's really noticeable with 7200 RPM and slower drives. It usually leads to the whole system crawling for a few minutes after desktop shows up. It would be great if the OS would measure the different loads and UI response times, and actually limit the startup programs. This way you could open your browser and other tools and those would be given priority upon startup process.
I tested the developer preview version briefly and it sure seemed a lot faster and snappier. The startup time is remarkably faster. And according to this PCMag test, seems like the overall speed has been improved a lot too. Good job MS!