The siege on SEI's bases took over three hours, and that was with a force of 7-8 Valors present and almost entirely undisturbed for the bulk of the siege. That was for two Core 2 bases, one with all three repairs and the other with either two or three.
There's no way a fully stocked Core 5 gets flattened in less time than a Core 2, especially not by a ragtag bunch of bad Ranseurs busy getting hammered by bombers and caps the whole time.