Conversion from STL to STEP is slow

I am trying to convert an STL file into and STEP file. The file size id around 4MB. This is the following code that I use to perform the operation:

Standard_Boolean StlAPI_Reader::Read(TopoDS_Shape&          theShape,
	const Standard_CString theFileName)
{
	Handle(Poly_Triangulation) aMesh = RWStl::ReadFile(theFileName);
	if (aMesh.IsNull())
	{
		return Standard_False;
	}

	TopoDS_Vertex aTriVertexes[3];
	TopoDS_Face aFace;
	TopoDS_Wire aWire;
	BRepBuilderAPI_Sewing aSewingTool;
	aSewingTool.Init(1e-6, Standard_True);

	TopoDS_Compound aComp;
	BRep_Builder BuildTool;
	BuildTool.MakeCompound(aComp);

	const TColgp_Array1OfPnt& aNodes = aMesh->Nodes();
	const Poly_Array1OfTriangle& aTriangles = aMesh->Triangles();
	for (Standard_Integer aTriIdx = aTriangles.Lower();
		aTriIdx <= aTriangles.Upper();
		++aTriIdx)
	{
		const Poly_Triangle& aTriangle = aTriangles(aTriIdx);

		Standard_Integer anId[3];
		aTriangle.Get(anId[0], anId[1], anId[2]);

		const gp_Pnt& aPnt1 = aNodes(anId[0]);
		const gp_Pnt& aPnt2 = aNodes(anId[1]);
		const gp_Pnt& aPnt3 = aNodes(anId[2]);
		if ((!(aPnt1.IsEqual(aPnt2, 0.0))) && (!(aPnt1.IsEqual(aPnt3, 0.0))))
		{
			aTriVertexes[0] = BRepBuilderAPI_MakeVertex(aPnt1);
			aTriVertexes[1] = BRepBuilderAPI_MakeVertex(aPnt2);
			aTriVertexes[2] = BRepBuilderAPI_MakeVertex(aPnt3);

			aWire = BRepBuilderAPI_MakePolygon(aTriVertexes[0], aTriVertexes[1], aTriVertexes[2], Standard_True);
			if (!aWire.IsNull())
			{
				aFace = BRepBuilderAPI_MakeFace(aWire);
				if (!aFace.IsNull())
				{
					BuildTool.Add(aComp, aFace);
				}
			}
		}
	}

	aSewingTool.Load(aComp);
	aSewingTool.Perform();
	theShape = aSewingTool.SewedShape();
	if (theShape.IsNull())
	{
		theShape = aComp;
	}
	return Standard_True;
}

It took me around 13 hours to complete the operation. I have tried changing the parameter aSewingTool.Init(), but there is no effect. 

On debugging I found out that most of the time is spent on the command aSewingTool.Perform().  

Is there any way to make it slower or any other way to convert it. 

Thanks.