Kernel task appears to be making my computer slow

Question

Lately Kernel_task has been appearing a lot in the group of first 5 applications using memory, and immediately making my computer slower, and even getting the internal ventilator make a lot of noise because of the extra work. I am working on the Startup disk space, getting rid of things to make space (what would be the ideal minimum space I should keep unused?), but I don't know if this is related with the kernel_task
Hope I can get any hint on how can I fix this, hopefuly by myself.
Hardware Information:
    MacBook Air (13-inch, Mid 2011)
    MacBook Air - model: MacBookAir4,2
    1 1.7 GHz Intel Core i5 CPU: 2 cores
    4 GB RAM
Video Information:
    Intel HD Graphics 3000 - VRAM: 384 MB
System Software:
    OS X 10.9.2 (13C1021) - Uptime: 0 days 7:53:2
Disk Information:
    APPLE SSD TS128C disk0 : (121.33 GB)
        EFI (disk0s1) <not mounted>: 209.7 MB
        Macintosh HD (disk0s2) / [Startup]: 120.47 GB (3.81 GB free) (Low!)
        Recovery HD (disk0s3) <not mounted>: 650 MB
USB Information:
    Apple Inc. FaceTime Camera (Built-in)
    Apple Inc. iPhone
    Apple Inc. BRCM20702 Hub
        Apple Inc. Bluetooth USB Host Controller
    Apple Inc. Apple Internal Keyboard / Trackpad
    Apple Internal Memory Card Reader
Thunderbolt Information:
    Apple Inc. thunderbolt_bus
Gatekeeper:
    Mac App Store and identified developers
Kernel Extensions:
    [kext loaded]    org.virtualbox.kext.VBoxDrv (4.1.2) Support
    [not loaded]    org.virtualbox.kext.VBoxNetAdp (4.1.2) Support
    [not loaded]    org.virtualbox.kext.VBoxNetFlt (4.1.2) Support
    [kext loaded]    org.virtualbox.kext.VBoxUSB (4.1.2) Support
Startup Items:
    HWPortDetect_Cus: Path: /Library/StartupItems/HWPortDetect_Cus
    VirtualBox: Path: /Library/StartupItems/VirtualBox
Problem System Launch Daemons:
    [failed]    com.apple.wdhelper.plist
Launch Daemons:
    [loaded]    com.adobe.fpsaud.plist Support
    [running]    com.aladdin.eTSrv.plist Support
    [loaded]    com.oracle.java.Helper-Tool.plist Support
    [loaded]    com.oracle.java.JavaUpdateHelper.plist Support
    [loaded]    com.safenet.SfntMonD.plist Support
Launch Agents:
    [running]    com.aladdin.PKIMonitor.plist Support
    [loaded]    com.oracle.java.Java-Updater.plist Support
User Launch Agents:
    [loaded]    com.facebook.videochat.[redacted].plist Support
    [not loaded]    org.virtualbox.vboxwebsrv.plist Support
User Login Items:
    iTunesHelper
    Dropbox
Internet Plug-ins:
    Silverlight: Version: 4.1.10329.0 Support
    FlashPlayer-10.6: Version: 13.0.0.214 - SDK 10.6 Support
    Flash Player: Version: 13.0.0.214 - SDK 10.6 Support
    QuickTime Plugin: Version: 7.7.3
    JavaAppletPlugin: Version: Java 7 Update 55 Check version
    Default Browser: Version: 537 - SDK 10.9
Safari Extensions:
    Cuevana Stream: Version: 4.4
    Open in Internet Explorer: Version: 1.0
Audio Plug-ins:
    BluetoothAudioPlugIn: Version: 1.0 - SDK 10.9
    AirPlay: Version: 2.0 - SDK 10.9
    AppleAVBAudio: Version: 203.2 - SDK 10.9
    iSightAudio: Version: 7.7.3 - SDK 10.9
iTunes Plug-ins:
    Quartz Composer Visualizer: Version: 1.4 - SDK 10.9
3rd Party Preference Panes:
    Flash Player  Support
    Java  Support
Time Machine:
    Time Machine not configured!
Top Processes by CPU:
        37%    firefox
         6%    WindowServer
         2%    sysmond
         1%    plugin-container
         1%    eTSrv
Top Processes by Memory:
    1.21 GB    firefox
    66 MB    Finder
    45 MB    Skype
    41 MB    WindowServer
    25 MB    mds_stores
Virtual Memory Information:
    41 MB    Free RAM
    927 MB    Active RAM
    886 MB    Inactive RAM
    1.06 GB    Wired RAM
    2.04 GB    Page-ins
    151 MB    Page-outs

Answer 1

I did, it, here the print screen of what it's my usual work with Firefox (two windows, several tags). Did disable the extensions to make the test and these are the results.