The thing with fire spread is that it's not as straightforward as 'visible flames'. The fire was likely already raging heavily inside the building in the first picture, but was being contained within the roof. Eventually a portion of the roof gives way, with a domino effect bringing the rest of the roof down, finally releasing all of the smoke and flames that had previously been contained. Not to mention that once the roof gives way there's suddenly loads of air available for combustion, whereas the fire was likely being starved before.
That's not to say it wasn't insanely fast, though, and just goes to show why engineers (like me!) work very hard on fire systems within all kinds of buildings.