Where clause vs. axis . Performance difference is too big. Why?

I have 3 MDX-Queries

select{[Measures].[FactCount]}on 0,{[Dim2].[Field3].&[Dr]}*head([Dim2].[Field1].members, 100)on 1from[Fact1]select{[Measures].[FactCount]}*{[Dim2].[Field3].&[Dr]}on 0,head([Dim2].[Field1].members, 100)on 1from[Fact1]where[Dim2].[Field3].&[Dr]select{[Measures].[FactCount]}on 0,head([Dim2].[Field1].members, 100)on 1from[Fact1]where[Dim2].[Field3].&[Dr]

The result is basically the same, but the perfomance of the 3-rd Query is 1/1000 of the 1-st und 2-nd queries.

What does the matter?