The shrapnel is preceded by a blast wave travelling around say mach 4. This is very high pressure, so for a moment there is higher pressure outside, so it doesn't decompress. Then the wave passes, it decompresses by rolling and curling back the panels along the perforations. It does not need to be explosive to rip up the cabin, because of the shrapnel holes. Twisting, banking, awing, diving, parts ripping of, port engine ingesting its own fan.... a lot of G forces in a lot of directions it was never designed for ....enough to break along ALL the section join lines. Fast or slow,who really will ever know? but there's only 2 or 3 seconds difference between the two speeds, so its moot.