Version: (using KDE KDE 3.4.0) Installed from: Gentoo Packages Kmail GUI freezes for a long time when you select many messages from a folder and moves them to another, but only if one of the folders is in mbox format. maildir <-> maildir moves are quite fast so the GUI blocks anyway but just for less time. It took 2+ hours to move 3000 messages from mbox to maildir. You should remove the GUI freeze and make possible to stop the process.
The move is very slow even if both dirs are fully contained in ram (no disk activity).
(KDE 3.5.1, KMAIl 1.9.1) maildir<->maildir move is not that fast either. Moving 6309 messages between 2 NFS maildirs took several minutes and consumed considerable network bandwidth: Before: # ip -s link show dev lan1 2: lan1: <BROADCAST,MULTICAST,UP> mtu 6000 qdisc pfifo_fast qlen 1000 link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff RX: bytes packets errors dropped overrun mcast 272590096 213635 0 0 0 1 TX: bytes packets errors dropped carrier collsns 210820280 213657 0 0 0 0 After: # ip -s link show dev lan1 2: lan1: <BROADCAST,MULTICAST,UP> mtu 6000 qdisc pfifo_fast qlen 1000 link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff RX: bytes packets errors dropped overrun mcast 450667222 392535 0 0 0 1 TX: bytes packets errors dropped carrier collsns 363576848 392555 0 0 0 0 178MB read, 152MB write. Since message can be moved between maildirs on the same device with a simple rename, this is inefficient. For comparison: A simple 'mv' of 6309 files took 20 seconds, # ip -s link show dev lan1 2: lan1: <BROADCAST,MULTICAST,UP> mtu 6000 qdisc pfifo_fast qlen 1000 link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff RX: bytes packets errors dropped overrun mcast 459172546 419878 0 0 0 1 TX: bytes packets errors dropped carrier collsns 372514124 419896 0 0 0 0 isjsys:~ # ip -s link show dev lan1 2: lan1: <BROADCAST,MULTICAST,UP> mtu 6000 qdisc pfifo_fast qlen 1000 link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff RX: bytes packets errors dropped overrun mcast 465899264 451431 0 0 0 1 TX: bytes packets errors dropped carrier collsns 377347846 451449 0 0 0 0 = 6MB read, 5MB write (Yes, I know that KMail needs to update the index too, but still...)
Thank you for taking the time to file a bug report. KMail2 was released in 2011, and the entire code base went through significant changes. We are currently in the process of porting to Qt5 and KF5. It is unlikely that these bugs are still valid in KMail2. We welcome you to try out KMail 2 with the KDE 4.14 release and give your feedback.