[OpenSCAD] Minkowski sum slowness

[Giles Bathgate]

I have the following "rounded box" generated using openscads minkowski
sum feature:

$fn=50;

minkowski()
{
 cube([10,10,1]);
 sphere(r=2,h=1);
}

This takes 18 minutes to render:

Parsing design (AST generation)...
Compiling design (CSG Tree generation)...
Compilation finished.
Compiling design (CSG Products generation)...
Processing uncached minkowski statement...
..rendering time: 0 hours, 18 minutes, 39 seconds
Compiling design (CSG Products normalization)...
CSG generation finished.
Total rendering time: 0 hours, 18 minutes, 39 seconds

I guess the stock response will be that CGAL is to blame, but
something must be wrong because the following script:

$fn=50;

minkowski()
{
 cube([10,10,1]);
 cylinder(r=2,h=1);
}

takes negligible time:

Parsing design (AST generation)...
Compiling design (CSG Tree generation)...
Compilation finished.
Compiling design (CSG Products generation)...
Compiling design (CSG Products normalization)...
CSG generation finished.
Total rendering time: 0 hours, 0 minutes, 0 seconds

Regards

Giles