How do I update GtkProgressBar in a loop?

I can not simply call gtk_progress_bar_set_fraction while in a loop, because it will not take any affect until the loop ends.

If I use gtk_main_iteration(); to force main to iterate along the call of gtk_progress_bar_set_fraction it works of course.. but the loop processes significantly slower.

What can I do to update the progressbar while processing in a loop?