Code can be well designed from an architectural perspective and still be slow in areas. I imagine it's due to the minimal feature set (which MS defined). Stuff constantly happening in the background when you click or type something, bottlenecks become more apparent with really large projects and they just haven't dedicated time to improving that, etc.

I realize that architecture can't be separated from functionality, but I think the parent was focused on modularity and the ability to reuse various components.