Sql server What happens in delete with index : I have a heap table of 36 mill,am trying to delete selected 6 mill.. When i delete in 100 of batches reads is 16++++++ and write is 113 so i add NON-clus index on Identity column to make read faster, now read is 34899 and write is 3508 with adding one NON-clus Idx it increase the write IO shoot up.
So what to know why this much difference what does sql server does in behind as in plan i see table delete is 8% and no more details.

just to  add more before doing this I drop 2 Nvarchar column, and clean the table and rebuild it 