If you enchant a Pickaxe with Efficiency V and try to brake Packed ice (#0174) it brakes slowly, the same speed with any tool, enchanted or not.
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The Efficiency works on regular Ice(#0079) so i would think it should have the same effect on packed ice
According to the wiki, a pickaxe is the assigned tool for breaking both normal Ice and Packed Ice. If that information is correct, then Efficiency should work on both.
@Preben: I'm not trying to suggest a feature, I was (both here and at MC-11992) asking why it's intended. Often when a report is marked as Intended like that, a Mojangster gives a reason. Here, it's an annoying change, with no obvious reason for it, yet clearly intended.
I guess I will be making a suggestion though...
Efficiency is broken in general. See MC-65533
I would say Works as Intended, as efficiency only applies to blocks that the tool is normally faster at breaking. Packed Ice, like regular Ice, does not have a specific tool that is faster at breaking it, so efficiency will have no benefit.
Efficiency was changed back around 1.5 (I forget the exact version that changed it, but I think that's right) from its previous behavior, which would have produced the effect you are looking for.