A build-fs probably read the whole disk to find what it need to rebuild (I did not check inside but it sound like that). After, if you add that for reading a fatty 2TB hard drive still go at the speed of a 80GB drive, that will justify the time ;)
So if fsck go read the nodes "one at a time" and that you have something like a Western digital Grean 2TB, you end up with a nominal transfer speed of 56.6MiB/s with is about half the max because you miss one (slow)RPM of read, you end up with 609 minutes and so just to read the STORAGES TREE nodes...
If fsck read them "one at a time", it could be speed-up by doing a sequential read of the whole disk and using the ram for buffer to scan for theses nodes but it's not actually the priority (lot of thing are more important than the speed of fsck, for exemple FITRIM to keep our SSD fast).
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