Native c++ dll running slower when called from VB.net vs. running called from native .exe

I have some code in native C++ (Visual C++ 2010) to process a file of some GB. I compiled it to an .exe and it takes about 8 minutes. But I need to call it from a Visual Basic .net interface, so I put it in a .dll and created a c++/cli wrapper class to call my code in a native dll. The only interaction between the managed code and native dll is to call the function that initiates the processing. To my surprise the processing takes almost double the the time that takes the .exe way. I´m not really an expert in VB.net so maybe there are some settings or something to look at I don´t know. Any idea welcomed. Thanks in advance.