I've been having the exact same problem and have been testing a variety of configurations to pinpoint what the issue could be. Besides "using" the MBA, is there anything special about what you are doing?
For exampe, are you using a bluetooth keyboard and/or mouse? Are you using the USB Ethernet Adapter and/or a mini-DVI plug going to a monitor?
As a side note, check out this question: Performance "throttle back" problem
Brian Caslis' response: +"Yes, it's true if it gets really hot one core shuts off to reduce temperature."+
This may very well be what is happening. One core gets shut off and the system just doesn't know how to handle itself with a single core. That's a lame explanation, I know, but it does explain the issue. Although, there has to be something we can do, short of purchasing a notebook cooler, that will not cause this to happen every single time we are doing "a lot" of work.
I'm going to roll with my own explanation, it sounded lame at first, but I think I can verbalize it better.
I'm going to run with the theory that temperature is causing this issue. Heavy processing can cause the temperature of the machine to rise and thus, in an effort to save the core(s) the MBA shuts off one of the cores. This is a normal thing for CPUs, if a CPU gets "too" hot it must save itself from utter damage and shut itself off. This, unfortunately, seems to cause a vicious cycle: the shutting off of one core forces all the work on the remaining core, this means the temperature will not get back to normal on its own. The user is forced to close I/O or CPU intensive applications, this slowly brings the temperature down and allows the core that shut off to be turned back on.
Improper heat dissipation may add to the problem. If your MBA is closed (because you are using an external keyboard and bluetooth keyboard and mouse), heat cannot dissipate properly. The heat vents are on the bottom, in the rear of the MBA, perhaps they are blocked by something and thus adding to poor heat dissipation.
This is totally a theory. Any thoughts?
So, i'm stuck in the same situation: the air gets too slow to even keep up with keystrokes or mouse movements. Great!
One thing I have seen in the console is that syslog / launchd mysteriously fail on various tasks. See the log below:
Feb 24 10:35:09 personal-macbook-air com.apple.launchd (com.apple.syslogd): Exited abnormally: Bus error
Feb 24 10:35:38 personal-macbook-air com.apple.launchd (com.apple.syslogd): Exited abnormally: Bus error
Feb 24 10:41:08 personal-macbook-air com.apple.syslogd: syslogd(351,0xa0978fa0) malloc: * mmap(size=3606249472) failed (error code=12)
Feb 24 10:41:08 personal-macbook-air com.apple.syslogd: * error: can't allocate region
Feb 24 10:41:08 personal-macbook-air com.apple.syslogd: * set a breakpoint in mallocerrorbreak to debug
Feb 24 10:41:41 personal-macbook-air com.apple.syslogd: syslogd(351,0xa0978fa0) malloc: * mmap(size=3606249472) failed (error code=12)
Feb 24 10:41:41 personal-macbook-air com.apple.syslogd: * error: can't allocate region
Feb 24 10:41:41 personal-macbook-air com.apple.syslogd: * set a breakpoint in mallocerrorbreak to debug
Anyone else seen this? thoughts? The fact that the debug output calls for a true debugging session by apple engineers is worrying.
I can confirm I'm using an external monitor most of the time - but I'm on the road at the moment without one and it's still happening. I'm also without my usual bluetooth keyboard and mouse.
I liked the temperature theory so I installed a piece of software that monitors the temperature of the CPUs - while it gets high when the kernel_task process goes crazy, it's just as high at other times when the computer runs perfectly.
Apple, please help! There are quite a few people reporting this issue now.
I use a 24-inch external monitor with my MBA most of the time. I also have this kernel_task problem and it's annoying! I think i've narrowed down the cause to network issues. I almost always get it when interacting with my NAS/xferring files and sometimes when saving files to FTP. Maybe it comes down to a buggy Wi-Fi driver kernel extension?
I had the same problem on my MBA this morning. I had been using a 24" external monitor, but had disconnected it. Also, I was using the USB-EtherNet adapter. Sometime later, the kernel_task went to over 70% and the MBA was running on one CPU; it was almost unusable. I tried rebooting (took a while) and the problem persisted. After reading these posts, I unplugged the USB-EtherNet adapter and the kernel_task usage immediately dropped to normal, and the MBA started using both CPUs again. I turned on Airport, no problem, then turned it off, no problem, then plugged the USB-EtherNet adapter back in, no problem.
At this point, I cannot recreate the problem, but unplugging the USB-EtherNet adapter seems to have stopped it.
Right, some more information - I can confirm that when the performance degredation happens it's indeed followed by the stoppage of one of the cores (in Activity Monitor the bottom CPU usage box is black with no activity). Closing the lid and reopening it fixes the problem immediately.
The temperature monitor software has the temperature at around 65c the entire time, and the fans are not spinning much harder than normal.
Apple, please acknowledge this issue and do something about it! It happens a lot when time machine kicks in during normal operation.