I can imagine that the connection is throttled.
https://www.fastmail.com/help/files/davnftp.html
Quote:
We've had a couple of users doing reasonable things which just happen to hit pathological cases in our filesystem implementation. To stop this happening, we've had to restrict the FTP server to one connection per user. Hopefully we can remove this limitation again soon, but for now it's needed to ensure that the service keeps working smoothly for everyone else.
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