From another thread here somewhere... I have pasted this just above Worker_Threads option in my ioFTPD.ini
Code:
# Worker_Threads, Number of worker threads is dependant on the number of users you have online
# For 10 users, 2-3 is enough.
# For 20 users 4-6
# 100 users 10-15
# 1000 users 20-30
/ZR