Evolution periodically re-scans IMAP stores to check the folder tree.  Once
upon a time (before the Camel re-write?) it used to use a single 'LIST ""
*' command.  Now it issues 'LIST "" %', interprets the result, and adds
each discovered subfolder to a queue of pending 'LIST "" folder/path/%'
commands.

With an IMAP store containing many subfolders, this takes such a long time
I was beginning to think I must have left "Check for new messages in all
folders" enabled.  Evolution becomes unresponsive for unacceptably long
periods.

Any chance of selectively reverting to the old behaviour?  Or running the
scan in the background?  ('Selectively' because it seems the interests of
people with local IMAP stores containing a lot of junk, and those with slow
remote IMAP stores may conflict - see #4957.)