Terribly slow system. Tried everything.

Question

OK, so my wife has this Mid-2012 MBP which had Lion installed and worked without a flow, no problems of any kind. Then we downloaded Mavericks and her Mac became unusable, literally you couldn't listen to a song in Itunes without it getting interrupted every 3 seconds (literally!). I checked the Mavericks forums and found many others were having similar problems, so I thought that was the problem. Anyway, I tried to solve it with Mavericks first: Ran Apple Hardware Test, no problems found. Repaired disk, repaired permissions, resetted PRAM, resetted SMC, repaired keychains, uninstalled apps. Nothing, Unusable.
Since I had Mountain Lion and I actually use it in my own MBP without any issues, I decided to completely erase her disk and make a clean ML installation. After sorting out the ML installation problems (i.e., reset the PRAM so the installer can actually finish!) ML installed. I let Spotlight indexing all night long and then checked the system. While it's way better than how it was in Mavericks (I am currently typing this in her Mac) it's nowhere near acceptable. The system seems to suddenly hang with no reason. No program or process is seen in Activity Monitor when this happens and there's quite enough free RAM. While typing this, I had to wait 2 times already (a 1 minute pause first, then a 20 seconds pause) because of this behaviour. Itunes is able to play maybe 1 complete song but the pauses come sooner or later.
Needless to say I have tried everything once again: Repair disk, repair permissions (which for some reason don't get repaired, but I doubt this is the cause of problems), running manteinance (with Onyx), reset the PRAM, reset the SMC. You name it.
I am starting to think it COULD be a hardware issue (I still think is very suspicious that all problems started the moment we installed Mavericks) but now I can not run AHT because it seems is not installed and for some reason the internet recovery doesn't work. It didn't pointed any error when I ran it the first time, though. Anyway, I just want to ask if there's any other thing I could try before taking the MBP to an Apple Center, cause I have some not-so-good experience with Apple "geniuses" and many times I have solved problems when their answer was "replace the disk" or something like that. Thanks in advance for any suggestions.
Here's the Etrecheck results (since I know you will ask for it anyway) :
Hardware Information:
          MacBook Pro (13-inch, Mid 2012)
          MacBook Pro - model: MacBookPro9,2
          1 2.5 GHz Intel Core i5 CPU: 2 cores
          4 GB RAM
Video Information:
          Intel HD Graphics 4000 - VRAM: 512 MB
Audio Plug-ins:
          AirPlay: Version: 1.7
          iSightAudio: Version: 7.7.1
System Software:
          OS X 10.8.5 (12F45) - Uptime: 0 days 0:28:31
Disk Information:
          TOSHIBA MK5065GSXF disk0 : (500.11 GB)
                    disk0s1 (disk0s1) <not mounted>: 209.7 MB
                    Macintosh HD (disk0s2) /: 499.25 GB (369.3 GB free)
                    Recovery HD (disk0s3) <not mounted>: 650 MB
          MATSHITADVD-R   UJ-8A8 
USB Information:
          Apple Inc. FaceTime HD Camera (Built-in)
          Apple Inc. Apple Internal Keyboard / Trackpad
          Apple Computer, Inc. IR Receiver
          Apple Inc. BRCM20702 Hub
                    Apple Inc. Bluetooth USB Host Controller
FireWire Information:
Thunderbolt Information:
          Apple Inc. thunderbolt_bus
Kernel Extensions:
Problem System Launch Daemons:
Problem System Launch Agents:
Launch Daemons:
          [loaded] com.adobe.fpsaud.plist
Launch Agents:
User Launch Agents:
          [loaded] com.google.keystone.agent.plist
User Login Items:
          iTunesHelper
3rd Party Preference Panes:
          Flash Player
Internet Plug-ins:
          AdobePDFViewer.plugin
          AdobePDFViewerNPAPI.plugin
          Flash Player.plugin
          FlashPlayer-10.6.plugin
          JavaAppletPlugin.plugin
          QuickTime Plugin.plugin
User Internet Plug-ins:
Bad Fonts:
          None
Time Machine:
          Time Machine not configured!
Top Processes by CPU:
              2%          mdworker
              2%          WindowServer
              1%          EtreCheck
              0%          Google Chrome
              0%          SubmitDiagInfo
              0%          PubSubAgent
              0%          fseventsd
              0%          mds
              0%          SystemUIServer
              0%          Google Chrome Helper
Top Processes by Memory:
          82 MB              com.apple.quicklook.satellite
          78 MB              Google Chrome
          70 MB              Finder
          61 MB              Google Chrome Helper
          61 MB              mds
          41 MB              WindowServer
          41 MB              Dock
          29 MB              DashboardClient
          25 MB              coreservicesd
          25 MB              SystemUIServer
Virtual Memory Statistics:
          1.73 GB            Free RAM
          973 MB             Active RAM
          179 MB             Inactive RAM
          1.14 GB            Wired RAM
          383 MB             Page-ins
          0 B                Page-outs

Answer 1

You can reset user set (bold) network.http.* prefs on the <b>about:config</b> page via the right-click context menu to the default value.
*http://kb.mozillazine.org/Images_or_animations_do_not_load#First_steps
*http://kb.mozillazine.org/network.http.pipelining
Try to disable IPv6 (check for other possible causes as well):
*http://kb.mozillazine.org/Error_loading_websites#IPv6