After the update to 1.4.6 using an efficiency IV or V diamond pickaxe on grass or an efficiency IV diamond shovel on glass or glowstone does not affect the speed even though there is no report in the changelog that some bug related to this has been fixed or that the way enchanted tools work has changed in the update 1.4.6

In 1.4.5 and previous versions these tools would mine these blocks instantly on point.