Your server will be faster with a different FTP server. Probably because bots aren’t hammering the FTP server anymore.

Another solution is, block the FTP ports on the firewall, and use a SSH proxy to access FTP. The bots aren’t hammering SSH. Probably because password login is disabled.

Maybe it has non secure FTP disabled, that is port 20 and 21. Good idea, close those two ports. But, I’m lazy.

What port does TSL FTP use? It might still use port 20 or 21. The other ports open are for passive FTP.