Slow FTP connection behind F5

Hello there,

I have an issue I don't understand.

Some customers connect on one of our FTP server behind the F5.

Public IP of the virtual server on port 21 > default pool > one server port 21

No irule nothing.

It lasts nearly 30 secondes between the login and the listing of the FTP content.

But if I try to connect on the FTP server with it's private IP it's instant.

I am not on the same vlan as the FTP so it goes through several switchs and firewalls in private and public.

Any idea? Thank you !