With Efficiency V and Haste II applied, many blocks in the game are able to be instantly mined, however these blocks which have slab variants are not affected by these player mining buffs; stone and sandstone are a good example of this. Even double-slab blocks, which resemble normal blocks in all but blockstate data, are slower than the original material block.

To replicate:
1) Give the player a Netherite Pickaxe with Efficiency V.
2) Give the player the Haste II effect.
3) Set a line of blocks one block off the ground (at eye level) of stone blocks, of five to ten blocks total, and have the player walk into the line while mining in front of them.
4) Repeat the above step with stone slabs, either in bottom-slab state or double (or test both).
5) Note that conditions providing for instant-mine of full blocks are not capable of allowing the player to mine the related slab at the same speed.