Yet another question about the slowness of the Iceweasel browser

Using Iceweasel since three years on my Debian (today Wheezy), I noted that the browser became very slow since one month.

The slowness IS NOT when I open the application BUT ONLY for the page load in tab.

Average time for loading: 6 seconds minimum, often more than 15 seconds  
Cache cleared each time I test
Motherboard with bi-proc  
Basic page style
CPU load less than 5%  
The Iceweasel installed version is 31.4.0  

I tried several stuff but the phenomenon didn't disappeard.

1) starting without the add-ons

2) stopping some processes

3) stopping all virtual machines (kvm/libvirt)

4) disabling ipv6

Several releases of Debian were installed since one month: does this probably explain that ?

How could I escalade the problem ? with wireshark ?

Thank you by advance for any suggestions.