Very simple: the "slow" drive isn't really that slow, it just has a LOT more data on it. Look at the "block counts" rather than the file counts. The "slower" drive has almost double the amount of data stored on it, which may be enough to cause some "thrashing" in e2fsck (cannot hold all data structures in memory at once, so it has to reread some of them over and over ..).

Cheers