[HPsquared]

The number of tracks per platter increased, but the RPM didn't. Assuming the heads can only read one track at a time per platter, it will take (number of tracks per platter / rpm) minutes to read the whole disk. Therefore it will take longer to read the whole disk. QED

[bcrl]

Drives don't read from multiple platters in parallel, so it's actually num_tracks * num_platters * 2 / rpm to read in the best case. Real world is more than that since it takes time for the drive firmware to shift between different platters and tracks.

*edit: not entirely true: the Seagate MACH.2 drive series that has 2 separate head assemblies per drive. They're pretty expensive and hard to get ahold of.