Explorer is slow because it's extensible. All the thumbnails and extra stuff in the context menu is what slows it down. There's some pathological cases like "opening a folder with an MP3 file in" which puts it into scanning all the files for ID3 tags.
Fpilot is a great example of how it's possible to make something MUCH faster by limiting the feature set.
(also some of the NTFS APIs are horribly slow for extended information)
They can start by making explorer startup in less than 5 seconds. Let's face it, Microsoft devs don't care about performance and probably never will.