Minecraft wiki entry for Blocking, associated text: Blocking allows for the slowest movements possible. This can occur when the player blocks, sneaks, and moves backward while under the effect of the slowness potion while going through cobwebs placed above still water on top of soul sand that is placed on top of ice. This causes the player to move at about 1/200 blocks a second, or 2/25 pixel a second (1 Pixel every 12-13s) on a default 16x16 texture pack. Blocking while sneaking and with the additional events (but without slowness potion) allows the player go at roughly 0.023854961832 blocks per second (1 block every 41.92 seconds).‌[Java Edition only]
It is impossible to block attacks on the New Nintendo 3DS Edition, as both shields and sword blocking are absent from the game.