there is a noticeable delay when switching from message to message while in view mode within a sorted mailbox folder with :prev or :next (J or K with default keybindings).
this delay did not exist prior to e50ab5928475 ('sort: keep sort criteria applied to folder')
expected behaviour: load :next or :prev message quickly with the maildir backend.
problem: messages take as long and sometimes longer to load than with the imap backend.
steps to reproduce: use the maildir backend with a default unsorted config, run :sort (e.g. :sort -r date), view a message and attempt to view the next messages with :next (J).
there is no delay when attempting this on an unsorted maildir.