minkowski and hull operate on every vertex in your object mesh - this means they need to a) convert the solid into a mesh and b) do math for every combination of vertices in said mesh. That's O(n^2) operations for objects of complexity n. This means that they are much slower than csg operations (that operate on the underlying geometry rather than the mesh). They are definitely the slowest (but most powerful) operators in OpenSCAD. If you're doing something like minkowski-adding a sphere to a complex shape, use $fn=(some low number) on the sphere to reduce face count and speed things up initially while you're hacking around, then increase it again before final render.