I think i have finally found the reason for the extremely slow installation of build 10049:

While it was installing, i took a look at Task Manager.
At aproximately 33% there is a high network activity -- meaning it's downloading something in the background.
It then sits at 34% for about 5 or more minutes.

I decided to open the SoftwareDistribution folder to see what exactly is it downloading, i found several "lp.cab" files inside that folder.

I later confirmed this by running "lpksetup /u"...
There were various language packs installed besides the default one (portuguese-brazil).

So that's why it takes almost 6 hours, it's basically installing all of those LPs.
Could unplugging the network cable prevent it from doing this? Or would it just get stuck forever?