Cache and time lag

[eluser]richthegeek[/eluser]
The lag is from the server transfer and parsing by jQuery, not from the PHP.

Server transfers are never going to be as fast as you need them to be - the *fastest* I have ever seen is ~500ms and on average it's about double that. Regardless of how small the output is (500ms was with a <10 char string as the output) or what encoding you use, the transfer will always have a minimum required for DNS resolution, connection, acceptance of the response, and initial parsing by the javascript (pre-callback).

Long story short, you should be creating the tree structure on page output (ie, via PHP) and controlling just the collapsing/expanding with jQuery.