Macbook Air running extremely slow - kernel_task at 300% cpu usage

Question

I purchased my Macbook Air roughly 15 months ago and it's performance has constantly been impressive since I purchased it - until roughly a fortnight ago. I came home from work one afternoon and it was running at a snails pace. I had recently upgraded to Yosemite though I'm unsure if this had anything to do with it. I had also recently installed FitBit software to sync to my new FitBit flex.
It has continued to run in this manner constantly since the problem began. It is ridiculously slow, every single request takes a very long time to respond. Logging in takes forever. When I click on an app to open it, the icon bounces for 30 seconds or more, and it's not a smooth bounce either, it's quite a jagged bounce!
I'm hoping someone is able to provide some advice, guidance or help. The laptop is no longer covered by the 12 month Manufacturer Warranty, and I did not purchase an Apple Care plan as no Apple product I've owned before has experienced issues like this.
This whole experience is killing me! I'm suspecting hardware failure and I'm just so sad that this could happen to the laptop I've loved so much since I bought it!
The problem:
The kernel_task process is constantly running at 300% or higher cpu usage, with Activity Monitor indicating the processor is being maxed out by a system task. Also, the laptop's fan is constantly running at full capacity - from the moment the laptop turns on, or wakes up.
What I think the problem might be:
As a result of many, many Google searches, it appears that this behaviour may be due to the Macbook Air thinking it is overheating - and I will clarify here that it certainly is not. It is cool to touch, always, when this problem occurs. (yet the fan is almost taking off!). As a result of the MacBook Air thinking it's overheating, it not only is running the fan at a very high RPM (confirmed with iStat), but it also is intentionally choking the system with 'simple requests' in attempt to cool the computer down. Apparently this is a built-in feature of OS X. The kernel has priority over user tasks, causing all user tasks to be granted a lower priority, causing the computer to run INCREDIBLY SLOW. Even as I'm typing this, I'm about two sentences ahead of what is showing on the screen!
Steps I have taken:
- Read widely across the internet with every possible relevant search term I can think of
- Downloaded iStat to see if any of the temperatures are too high (they all seemed fine..)
- Disabled Bluetooth and Wifi. (no change)
- Reset SMC (multiple times.., no change)
- Reset PRAM (multiple times.., no change)
- Drained the battery completely (2 or 3 times, no change)
- Install Windows 7 on BootCamp in attempt to bypass the issue (CPU usage was 100% constantly in Windows also)
- Uninstalled FitBit software (no change..)
- Uninstalled other software I did not need (no change..)
- Attempt to boot into Safe Mode / Recovery Mode (even in this modes, the same thing is happening)
- Repaired Disk Permissions
- Backed up all personal data
- Used the built-in Internet Recovery mode to format my Macintosh HD and re-install a fresh copy of OS X (Mountain Lion). (no change…)
- Re-upgrade to Yosemite (no change..)
I've downloaded EtreCheck and here is the output:
Problem description:
OS X is running incredibly slow. kernel_task process is constantly using 300% and upward CPU usage.
EtreCheck version: 2.1.5 (108)
Report generated 20 December 2014 3:38:49 PM AEST
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Hardware Information: ℹ️
  MacBook Air (11-inch, Mid 2013) (Verified)
  MacBook Air - model: MacBookAir6,1
  1 1.3 GHz Intel Core i5 CPU: 2-core
  4 GB RAM
  BANK 0/DIMM0
  2 GB DDR3 1600 MHz ok
  BANK 1/DIMM0
  2 GB DDR3 1600 MHz ok
  Bluetooth: Good - Handoff/Airdrop2 supported
  Wireless:  en0: 802.11 a/b/g/n/ac
Video Information: ℹ️
  Intel HD Graphics 5000 - VRAM: 1024 MB
  Color LCD 1366 x 768
System Software: ℹ️
  OS X 10.8.5 (12F45) - Uptime: 1:24:6
Disk Information: ℹ️
  APPLE SSD SD0128F disk0 : (121.33 GB)
  disk0s1 (disk0s1) <not mounted> : 210 MB
  Macintosh HD (disk0s2) / : 120.47 GB (106.42 GB free)
  Recovery HD (disk0s3) <not mounted>  [Recovery]: 650 MB
USB Information: ℹ️
  Apple Inc. BRCM20702 Hub
  Apple Inc. Bluetooth USB Host Controller
Thunderbolt Information: ℹ️
  Apple Inc. thunderbolt_bus
Gatekeeper: ℹ️
  Mac App Store and identified developers
User Login Items: ℹ️
  uTorrent Application (/Applications/uTorrent.app)
Internet Plug-ins: ℹ️
  QuickTime Plugin: Version: 7.7.1
  JavaAppletPlugin: Version: 14.6.0 - SDK 10.8 Check version
3rd Party Preference Panes: ℹ️
  None
Time Machine: ℹ️
  Time Machine not configured!
Top Processes by CPU: ℹ️
      17% WebProcess
      16% storeagent
      15% Safari
      9% WindowServer
      9% repair_packages
Top Processes by Memory: ℹ️
  404 MB repair_packages
  395 MB WebProcess
  129 MB Safari
  86 MB App Store
  64 MB Finder
Virtual Memory Information: ℹ️
  1.77 GB Free RAM
  1.76 GB Active RAM
  140 MB Inactive RAM
  623 MB Wired RAM
  425 MB Page-ins
  0 B Page-outs
Diagnostics Information: ℹ️
  Dec 13, 2014, 09:47:32 PM /Library/Logs/DiagnosticReports/Kernel_2014-12-13-214732_[redacted].panic [Details]
  Dec 13, 2014, 08:31:50 PM /Library/Logs/DiagnosticReports/Kernel_2014-12-13-203150_[redacted].panic [Details]
  Dec 13, 2014, 04:54:18 PM /Library/Logs/DiagnosticReports/Kernel_2014-12-13-165418_[redacted].panic [Details]
  Dec 13, 2014, 07:05:03 AM /Library/Logs/DiagnosticReports/Kernel_2014-12-13-070503_[redacted].panic [Details]
  Dec 20, 2014, 02:15:38 PM Self test - passed

Answer 1

Hi,
I have an update so I thought I would share in case any other users are experiencing a similar problem.
I made an appointment with the Genius Bar at the Apple Store.
I went in and they ran a thorough diagnostic check on my Macbook Air and determined that the sensors were indeed faulty - several were providing no information "No reading" instead of providing a temperature. Thus, the Air was unaware of the temperature across multiple components.
Despite the laptop being outside the 12 Month Manufacturer Warranty, the Apple Store determined that it was covered under warranty through Australian Consumer Law - the laptop is now in for a complete repair under warranty at no cost to myself.
I couldn't be happier.