I'm not sure if this is DOpus, or Vista. If a subfolder has a large number of items, e.g. folders, expanding the subfolder root in the Folder Tree can take quite a number of seconds. This appears to happen only the first time, thereafter it is almost instantly. Until the next reboot.
Basically I'm trying to speed it up a bit.
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You may be able to slightly speed up opening branches in the tree by turning off things like showing Zip files in the tree. In reality I doubt it will make much difference. The time it takes is going to be mostly the time it takes the OS and filesystem to read in the list of directories.
Unless something unusual is happening you should see the same sort of speed the first time you open the same folder in Explorer's tree, or any other program's, assuming you haven't recently opened it in another program.
The reason it's fast the second time you open it is that the OS / filesystem have cached the information.