Just tested pffexport and the library itself on PST with two folders, each of the folder contains about 250000 items. Waited for about 20 minutes on quad-core CPU (just to open the file) and killed the task. It looks like the library builds the entire node tree on file open and this process is extremely slow. Not sure why don't you build the tree partially by request (on get folder(s))...
Just tested pffexport and the library itself on PST with two folders, each of the folder contains about 250000 items. Waited for about 20 minutes on quad-core CPU (just to open the file) and killed the task. It looks like the library builds the entire node tree on file open and this process is extremely slow. Not sure why don't you build the tree partially by request (on get folder(s))...