Cobweb is not mined faster with Efficiency shears

Blocks such as wool and vines are mined faster using a pair of shears enchanted with Efficiency. However, Efficiency on shears doesn't make mining cobwebs any faster.

How to reproduce

1. Enchant a pair of shears with Efficiency V, either with an anvil or using the /enchant command. Prepare an unenchanted pair of shears as the control group.
2. Place two cobwebs. Switch to Survival mode, and mine one cobweb using each pair of shears.
3. Notice how the two cobwebs are broken in the exact same amount of time.