Not sure if a bug or a design decision, but it seems that combined actions always result in a 110% increase, even if the individual items are much more.
So if you want to go fast, you have to expand the items and click them individually.
The combined actions always give a 10% speed boost, which multiplies the speed of the combined actions. The tooltips will be improved next version to make this more clear, but you can prove it now if you add up the "modified* time of the sub actions and compare it to the "modified" time of the combined action.