The treemenu has been optimized a lot since the initial version. However, it was not designed to cope with too many nodes. That is why you have the possibility to turn it off and/or tweak it so that it only shows specific type of nodes. In addition, you can also limit it to only get a certain number of nodes and/or to only go to a certain depth within the node-tree. If you have a fast server and a fast connection: sure, it will probably work with a lot of nodes. If not: tweak/limit or or turn it off. :-)
Thanks for the hint! We're in some intranet, which usually has enough bandwith. I am more concerned about the DB queries, than about the HTML file size, that results, but any compression should help here, too.