httpwebwitch - 1:53 pm on Jan 16, 2013 (gmt 0)
If you're preloading the data to populate the dropdowns, then I'd define the dependencies using JSON, not XML. Just make a big object and then access parts of the object to create your <option> nodes.
In the case you're doing AJAX to get the dropdown contents, reading and grabbing data from a large XML file will be slower than getting the same info from a similarly-sized SQL table.
In either case, caching the AJAX output in something like Redis or Memcached will provide speed.