If I were you I would slightly increase the number of worker threads. The working fibers have disappeared a few betas ago btw.
Here is the configuration of my test server. There are rarely more than two concurrent users on this one so you might want to use more worker threads.
Code:
[Threads]
Process_Priority = NORMAL # Suggested process base priority (Idle/Normal/High/Realtime)
Io_Threads = 2 # Number of io threads
Worker_Threads = 5 # Number of worker threads
Encryption_Threads = 1 # Number of encryption threads
WindowName = ioFTPD::MessageWindow
darkone gave an explanation of all the types of threads and how you should configure them for optimal performance:
http://www.inicom.net/forum/showthread.php?t=13970