c++ - Visual studio 2013 platform toolset v120 performance is slower then when set to v100 -

We recently switched from Visual Studio 2010 to Visual Studio 2013, we are running a mathematical algorithm after a few measurements, It seems that the fast toolset = platform toolset = v100 (in Visual 2013) time running 7-15% V120 , does anyone know about it, we expected that v120 will not be reality and slow?