Comment 15 for bug 370999

As it is stated there the problem seems to be the plugin used.
I also had this problem, as it was using wodim to burn the iso instead of growisofs.
To remove wodim is not possible in a clean way as Brasero is marked to depend on it and it will be removed along with wodim.

But there is a quick and simple workaround to fix half of this problem.
Using Gnome Configuration Editor navigate too:

apps > brasero > config > priority

In there it list all the plugins found and their priority, by default all are in 0 so Brasero chooses the one that fit the best.

Two things can be done here either disable the plugin in this case wodim and that is done by changing the value to '-1' or
to increase the priority of the one we want and i went for growisofs and in that case giving it a value higher than 0.

In my case I did both:

growisofs-burn = 1 (raises the priority forcing it to be choose)
wodim-burn = -1 (disables it)

This fixed my problem regarding the wrong percentage been showed as soon as it reaches the 2GB mark as for the burning speed is still slow even while using growisofs.