It does its job very well, and if I wanted a menu system for a frameless site I'd chosen it. If one wanted to show one document's relation to another one wouldn't use the menu system at all for navigation, but instead choose other approaches. Speed Speed is, and will be, an important issue. The user wants quick access to information and therefore the script has to be as small as possible and show the content to the user as quickly as possible. This isn't an easy task since you also want features and configurability.
Features add size, making the script load more slowly.
One things that's important is also that once the script is loaded it has to provide the output to the user without too much delay. A layer-based menu is slow on startup since it has to write the whole menu to the document. If you have a large menu, and the script is not optmized well you can end up with a huge delay on startup. A menu system like mine will be quicker upon startup since it only writes to the document what is needed. If a sub-menu is closed we don't write the contents of it.