I didn't say they move independently. Just correcting your "1 head for 2 platters", "more surface area to cover" statement. If each platter surface has a head respectively, then each head covers the same platter surface area irrespective of the number of platters!
You seem sort of confused. If there is a data queue, how does it matter which platter the data is on? Even if queued data is on platter surface 1 and head 1 is busy with a data transfer on platter 1 , the queue still needs to wait for that task to finish before it gets fetched. The only possible lag I see is in the controller electronics sswitching between read/write head, and electronics isn't slow. If you don't find rotational speeds to have any difference, I'm surprised how you can find electronics to be slower
the reason considering all other parameters same,why more platter hdd will be slow, is as you said switching between read/write head, positioning of another platter if part of same file is spread across multiple platters.
it is not electronics, it is the actual mechanical movement that is slow.
