When clicking on a mail in kmail it takes very long until the mail is displayed. I had a look at it with the akonadiconsole and watched the kmail2-xxxxxx tab in the debugger.
I have some logs I'm willing to send you but I don't think it's a good idea to post them here.

It seems that especially these queries are slow:
189 OK SEARCH completed 
190 SEARCH "prefix nco:<http://www.semanticdesktop.org/ontologies/2007/03/22/nco#>SELECT DISTINCT ?person WHERE { graph ?g { ?person <http://akonadi-project.org/ontologies/aneo#akonadiItemId> ?itemId . ?person a nco:PersonContact ; nco:hasEmailAddress ?email . ?email nco:emailAddress \"xxxxxxxxxxx@gmail.com\"^^<http://www.w3.org/2001/XMLSchema#string> . } }" FULLPAYLOAD EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 

They sometimes show up multiple times, and it seems the more often they show up the longer the retrieval takes.
And sometimes email addresses show up that don't seem to have anything to do with the email that's being fetched.

Also the email headers seem to be fetched twice.
And doing a right click on an email to bring up the context menu takes very long, too.

Well, I really don't know how I can provide you with more useful information so let me know If you need something.

In KDE 4.7 a restart of akonadi fixed that problem but in KDE 4.8.1 this doesn't seem to work anymore.