FTPES is extremely slow

File transfers, especially downloads, are incredibly slow using the built-in FTP server when TLS is enabled. Using alternative servers in jails (vsftpd, pure-ftpd) does not exhibit this behavior. This happens in 11.3-U5 and was attested by a forum user to happen in the TrueNAS 12 beta.

https://www.truenas.com/community/threads/my-freenas-has-real-slow-speed-with-explicit-ftp-over-tls.86659/

(It is not clear to me if this is still a problem in 12-U1.1; have been holding off upgrading for unrelated reasons.)