I tried to solve this by storing the expanded section's id in a cookie and while re-generating the tree view, I check the sections against the id's stored in the cookie and expand those. But this turned out to be slow.

Have you tried to determine why it is slow? If the tree isn't particularly big—and I'm guessing it can't be if it is to remain useful to the user—then it would be hard to imagine this being noticeably slow. It's not like it would require any heavy computation.

One suggestion I'd make, though not for reasons of efficiency, would be to store your section IDs locally (i.e. in a session) and then pass only the key to the session via the cookie. Putting lots of stuff in cookies isn't scalable.

-sauoq
"My two cents aren't worth a dime.";