In a project we have a large menù (more than 400 links in 4 levels), foreach link a call to database is done to get the path alias of the link; all calls takes approximately 500 ms.
Proposed resolution
Use Ajax to load sublevels preloading the next level
It is interesting feature, but I am afraid, not a top priority for the moment. I will try to investigate it later. I am just afraid it could be super buggy