Apologies, the options for what is displayed in the tree moved into a slightly different place in the recent beta versions. If you're using 10.2.0.0 or earlier, they're under Preferences / Folder Tree / Appearance (sounds like you found it already, though).
Do they still open quickly after the first access even if you close the window and open a new one?
If they are still quick even in a new window, what if you fully exit Opus and then try again? Still quick, or then slow again?
Knowing that may give us a hint about where the information is being cached, which might in turn make it easier to work out where the delay is coming from.
It may also be worth running Process Monitor to see what dopus.exe is accessing during the delays. If you capture the activity that happens while the folder tree is expanding and then save that as a .PML log file, zip it, and send it to me as a private message, I'll take a look and see if anything stands out.