Michael,
The "generate a hollow object" is a difficult problem in general.

But if you want to print the hull of an object, it may be easier to just set
"infill" to zero on your slicing program.

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zero infill is not an option. the object is going to be parametric. I'll have
a look at the other optiions tomorrrow. thanks for the replys so far.

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@peter
what for is the //small number "e"?

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@peter, i understand the small number now but in general its not the solution
im looking for. let me refrase the qusetion:

difference(){

cube([50-x,50-x,100]);
x=2;
translate([x/2,x/2,0])
cube([50-x,50-x,100]);
}

if I could make it work that the 2nd cube gets another x value then the 1st,
i could reuse the same code for the inner and the outer object. Because we
are talking about a very complex object, this would help me a lot.

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Please note that in your program x is defined as 2 in all the code.
Openscad makes one pass through the code to process all the variabes.
Then it resolves all the variables and calculations and  writes all the
geometry in a CSG-tree.
For your code example this is:
(

group() {
difference() {
cube(size = [48, 48, 100], center = false);
multmatrix([[1, 0, 0, 1], [0, 1, 0, 1], [0, 0, 1, 0], [0, 0, 0,
1]]) {
cube(size = [48, 48, 100], center = false);
}
}
}

Then it sends this CSG-tree to the , CGAL http://cgal.org
engine to render. This last step is what takes so long.

So optimising the calculations in your code is almost always not effective.
The time to calculate the 3d-mesh takes way longer.