Load big file with STEPControl_Reader cost too much time

I load my step code(400Mb) with the code:

STEPControl_Reader step_reader;
step_reader.ClearShapes();
IFSelect_ReturnStatus status = step_reader.ReadFile(argv[1]);

if (status != IFSelect_RetDone)
{
std::cout << "read" << argv[1] << "fail！" << std::endl;
return 1;
}

int num = step_reader.NbRootsForTransfer();
for (int i = 1; i <= num; i++) step_reader.TransferOneRoot(i);
TopoDS_Shapee = aShape = step_reader.Shape();

However, it cost 4mins for "ReadFile" , and 10mins for "TransferOneRoot".
I have no idea how can my code cost so much.