Hello,
The FTP directory listing is a pain in the newest (as of today) version of IMSCP - 1.2.17
It takes an agonizing average of 60 seconds for the directories and files to simply get listed in an FTP client (in my case FileZilla).
I've checked the proftpd.conf for the MasqueradeAddress setting to see if it was to blame, but this setting was absent in any proftpd.conf file I have on the system (Debian 8.3 Jessie). I also have SSL enabled for panel and all services alike (TLS for ProFTPd) but I don't think that's related. I've already submitted an issue in the imscp bug report system and would hopefully love to see it fixed once and for all.
Martini