My love affair with the new OS X El Capitan has ended. Apple Mail problems slow down the iMac to a crawl. The problems are related to Kernel_Task.
So I just installed Thunderbird because Apple Mail is rendering my iMac unusable when running. The first impression of El Capitan was great. As reported, browsers felt faster. Now the initial positive experience as turned into a negative one. Since the weekend both of my iMacs have been having severe performance issues. The source of the problem is Apple Mail. When Mail is running the CPU load of the infamous Kernel_Task is spiking. This makes everything sluggish and after a couple minutes make the iMac completely unresponsive.
The issue happens on an iMac11, 3 and iMac12,2. On the older iMac (2010) the issue started already less then 24 hours after the El Capitan update. Now my newer iMac (2011) has the same issue.
I only have one IMAP account in Apple Mail. I have taken measures server side to reduce the amount of emails Mail has to deal with. This has not relegated the issue though. I have also tried to delete the IMAP account and the Mail folder under Library. This triggers a fresh sync, but also that did not fix the situation.
I also tried the mysterious Kernel_Task fix that requires to delete a model specific entry in the extension IOPlatformPluginFamily.kext. This fix described by Rhys Oxenham already back in 2012, has fixed a misbehaving Kernel_Task successfully for many users. This fix is though messing with the temperature control of your Mac. This can damage your hardware.
In any case the Kernel_Task fix has not resolved my issue. I definitely can link the runaway Kernel_Task issue with Apple Mail. When Mail is not running both iMacs have no issue.
There are several reports of Mail problems in El Capitan including crashes. I could not find any reports where users made the connection to Kernel_task.
The strange issue is that this problem is not appearing right away after the El Capitan upgrade. It starts to happen up to 3 days later.
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El Capitan has already a high profile problem with Microsoft Outlook. There is also a big issue with Apple's own email client. I hope the first El Capitan update will fix the Mail app and Kernel_task issues. Apple has already released OS X 10.11.1 as beta on September 29. Until then I keep using Thunderbird.