Version:           4.7 (using KDE 4.7.3) 
OS:                Linux

I use kmail with multiple imap accounts on a local mail server. One of those
accounts has a very large Trash folder (> 12.000 messages).

When working with kmail, it regularly stalls for minutes at a time in which you
can't do anything sensible with it. In most cases these stalls seem to be
because kmail is synchronizing the large Trash directory.

For example: I'm reading new mails in one imap folder (not the trash folder).
When moving to the next mail, the message preview window shows
"Mapinhoud wordt opgehaald
Even geduld..."
This is Dutch and roughly translates to
"Folder contents is being fetched
Please wait..."

This can take several minutes, after which the mail is finally shown.

Another example:
Sending a mail sometimes fails with the error:
"Er trad een fout op bij het in de wachtrij voor verzending zetten: Append
failed"
Which roughly translates as
An error occurred when adding to the send mail queue: Append failed"

Reproducible: Always

Steps to Reproduce:
1. Create an imap account with a very large trash folder (perhaps the same
happens with other folders as well, but it's the only one that's very big on my
system)
2. Just try to use kmail. I get very frequent stalls during normal usage.

Actual Results:  
Switching from one mail message to another can take minutes, sending mails fail
outright.

Expected Results:  
Switching from one mail message to another only happens with a minimal delay
(ideally less than a second, but minutes is definitely too long), sending mails
succeeds normally.

akonadiserver.error has multiple messages like this:
ItemRetrieverException :  Unable to retrieve item from resource: Did not
receive a reply. Possible causes include: the remote application did not send a
reply, the message bus security policy blocked the reply, the reply timeout
expired, or the network connection was broken.

I'm not sure if these messages are related though.

This is not a simple nuisance. The way kmail behaves on my system makes it
completely unusable for intense mail processing. I have currently switched to thunderbird, but can run additional tests if needed in kmail. I would really prefer to continue to use kmail.