@Obfuscator said:
I question if reading consecutive blocks is much faster, the heads just have to move a distance of 2x between every read instead of x, ...
Warning: some speculation follows.
Remember that we're talking about a spinning disc here. Since it moves, and since each track can hold more than one block, the heads don't have to move for every read or write. Much of the time, you can just hold still and just let the bits come to you as it rotates.
When you move a head, you also have to find the right block on a track. This means waiting for it to come to you, since the heads can't move in the "spinning direction" Presumably, in order to speed up reading consecutive blocks, moving from a track to its neighbour is optimized wrt waiting times, so that when you're done moving a head you only wait the absolute minimal time. (It also seems likely that you'd want to optimize the seeks themselves specifically for that case, but I don't know if that is possible.)
So for reading consecutive blocks; move occasionally (which is slow), and let the blocks of data come to you as much as possible. For "perfectly" interleaved blocks; move twice as many times and spend half of your optimal reading state waiting for blocks that are relevant to you. You might still be moving only track-to-neighbour (an x move) though.
If you're not dealing with perfectly interleaved files, then you'll instead have occasional longer seeks (very slow - large movement and long waiting times) and reading larger chunks of consecutive blocks (fast).