JFileChooser uses a background thread, BasicDirectoryModel.FilesLoader, to list the files and update the UI.

To determine whether a file is traversable, the thread calls filechooser.isTraversable(file) which is executed on the COM thread on Windows. https://github.com/openjdk/jdk/blob/0583f7357480c0500daa82f490b2fcc05f2fb65a/src/java.desktop/share/classes/javax/swing/plaf/basic/BasicDirectoryModel.java#L318-L320

That is for each and every file in list, there's a context switch between threads. This is very inefficient.

Before the fix for JDK-6713352, this piece of code was executed inside ShellFolder.invoke.