I refined my tests and it seems the check runtime is just a bit higher than the original model render time. The timings with a single sphere is unfair because the sphere alone is possibly rendered as a polyhedron without actual mesh building. To force the mesh building I just union the sphere with a cube. module model3() { sphere(10,$fn=100); translate([9,0,0]) cube(); } //check_model() model3(); model3(); There is something idiosyncratic about the module check_model(). I first tried to define it as the union of children() with the "void set" and it didn't work as planned. Why are union() and difference() dealt differently in this case? No clues.