[proftpd] Slow directory listing when using ftps

When I moved from sftp to ftps I experienced extremely slow directory listing (regardless of the client I used Cyberduck/Filezilla). I first suspected something was wrong with the passive mode (maybe it fell back to active mode or something) as I am behind a NAT router after all.

In the end it turned out to be the SPI firewall integrated in my Archer C8 router, so if you’re behind NAT and experience the same problems don’t forget to also check your integrated firewall.

PS This is why you never should use plain ol’ ftp in the first place.