Version:           Git (master) (using Devel) 
OS:                Linux

I've noticed KMail using 100% CPU and nothing was shown in statusbar. So I decided to see what is going on and I started the Akonadi Console. I switched to Debugger tab and enabled it. I could see a lot of messages like these being repeated:

KMail Kernel ETM (0x7f01f00042f0) * 58249 FETCH (UID 58249 REV 3 REMOTEID "{e2502026-9c0a-4e8f-8136-5967578d7cce}" MIMETYPE "message/rfc822" COLLECTIONID 5 SIZE 5752 DATETIME "09-Jul-2011 08:54:58 +0000" FLAGS (\SEEN) PLD:ENVELOPE[1] {467} ("Sun, 29 May 2011 06:12:44 +0000" "[Mageia-i18n] [Bug 1358] Mandriva mentioned in 33 package descriptions" (("andré blais" NIL "bugzilla-daemon" "mageia.org")) ((NIL NIL "mageia-i18n-bounces" "mageia.org")) (("List dedicated to internationalisation issues" NIL "mageia-i18n" "mageia.org")) ((NIL NIL "mageia-i18n" "mageia.org")) NIL NIL "<bug-1358-285@http.bugs.mageia.org/>" "<20110529061244.D2C0742B10@alamut.mageia.org>" "<bug-1358-285@http.bugs.mageia.org/>")) 
KMail Kernel ETM (0x7f01f00042f0) 73607 OK UID FETCH completed 
KMail Kernel ETM (0x7f01f00042f0) 73608 UID FETCH 85246 EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME PLD:ENVELOPE) 
KMail Kernel ETM (0x7f01f00042f0) * 85246 FETCH (UID 85246 REV 3 REMOTEID "{e250bc47-3a16-4a16-817b-bde1e88c2aa8}" MIMETYPE "message/rfc822" COLLECTIONID 5 SIZE 3888 DATETIME "09-Jul-2011 08:54:58 +0000" FLAGS (\SEEN) PLD:ENVELOPE[1] {397} ("Wed, 06 Jul 2011 10:46:15 -0700" "[meego-commits] 22313 accepted: MeeGo:1.2:oss/meego-ux-theme" (("Prajwal Mohan" NIL "no_reply" "build.meego.com")) ((NIL NIL "meego-commits-bounces" "meego.com")) ((NIL NIL "meego-packaging" "meego.com")) (("Prajwal Mohan" NIL "prajwal.karur.mohan" "intel.com")) ((NIL NIL "meego-commits" "meego.com")) NIL NIL "<20110706174615.8F19110A2A1@mail.meego.com>" NIL)) 
KMail Kernel ETM (0x7f01f00042f0) 73608 OK UID FETCH completed 
KMail Kernel ETM (0x7f01f00042f0) 73609 UID FETCH 47920 EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME PLD:ENVELOPE) 
KMail Kernel ETM (0x7f01f00042f0) * 47920 FETCH (UID 47920 REV 3 REMOTEID "{e250d3df-eb90-476b-b362-b33042aef787}" MIMETYPE "message/rfc822" COLLECTIONID 5 SIZE 5118 DATETIME "09-Jul-2011 08:54:58 +0000" FLAGS (\SEEN) PLD:ENVELOPE[1] {339} ("Tue, 10 May 2011 14:04:38 -0700" "[meego-commits] 17961: Changes to Trunk:Testing/dbus-glib" (("Chris Ferron" NIL "no_reply" "build.meego.com")) ((NIL NIL "meego-commits-bounces" "meego.com")) ((NIL NIL "meego-packaging" "meego.com")) ((NIL NIL "meego-commits" "meego.com")) NIL NIL NIL "<20110510210438.9AD1810A35F@mail.meego.com>" NIL)) 
KMail Kernel ETM (0x7f01f00042f0) 73609 OK UID FETCH completed 
KMail Kernel ETM (0x7f01f00042f0) 73610 UID FETCH 11836 EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME PLD:ENVELOPE) 
KMail Kernel ETM (0x7f01f00042f0) * 11836 FETCH (UID 11836 REV 3 REMOTEID "{e2517f13-f0a0-421a-b855-8b55c6dd1b01}" MIMETYPE "message/rfc822" COLLECTIONID 5 SIZE 3808 DATETIME "09-Jul-2011 08:54:58 +0000" FLAGS (\SEEN) PLD:ENVELOPE[1] {425} ("Fri, 04 Mar 2011 14:30:14 +0100" "[Bug 62697] 2011.0a2: Invalid D-BUS interface org.kate-editor.kwrite names" ((NIL NIL "bugzilla" "qa.mandriva.com")) ((NIL NIL "bugs-request" "mandrivalinux.org")) ((NIL NIL "cooker" "mandrivalinux.org")) ((NIL NIL "bugs" "mandrivalinux.org")) NIL NIL "<bug-62697-80605@http.qa.mandriva.com/>" "<bug-62697-80605-0IChxsnIv8@http.qa.mandriva.com/>" "<bug-62697-80605@http.qa.mandriva.com/>")) 
KMail Kernel ETM (0x7f01f00042f0) 73610 OK UID FETCH completed 

They were showing up quite slowly. What I found out was that if I switched in Kmail from displaying from Inbox folder to Outbox folder the messages started to come in quite a lot faster.

The Inbox has about 80000 emails and Outbox had none

Reproducible: Always

Steps to Reproduce:
1. Start Kmail and open a folder wit a lot of email
2. Check for new mail
3. Wait for KMail to use a lot of CPU
4. Open Akonadi Console in Debugger tab

Actual Results:  
x UID y FETCH EXTERNALPAYLOAD are slowly coming in unless switchine to a folder with very little email then they start to fly in very fast

Expected Results:  
x UID y FETCH EXTERNALPAYLOAD should be fast even if showing a folder with a lot of email

This is on a computer with 8 GiB of memory and Intel Core i5 2500K woth 4 cores