Can you please consider extending Claws Mail to support the RFC6851 MOVE extension? When moving messages between folders I always see the default IMAP COPY and delete sequence. Using MOVE on servers that support it (e.g. Dovecot (https://github.com/dovecot/core)) would be much faster if the server has an optimised implementation (e.g. when using Maildir and the server actually uses rename(2) or similar).