I think there's meant to be a 6 tick delay after mining a block before you can start mining another to prevent instant mining for most tools. If you mine a block, and there's no block within reach behind it, then the break delay never occurs and you can quickly aim at another block to start mining instantly, without the proper efficiency/haste combination.

One way you can do this is standing at the end of a row of blocks positioned just close enough that you can only reach the first block in the row. Then start sprinting towards it while mining. I've only tested with cobblestone and an Efficiency V diamond pickaxe.