The built-in Origin fitting function fits datasets with pre-compiled DLLs. If you are interested in improving the performance of your user-defined fitting function, you can create a fitting function using an external DLL. Fitting functions defined using an external DLL will work faster than those defined with Origin C.
We have prepared an Origin packing file, Fitting with External DLL.opx, in \Samples\Curve Fitting folder. After you drag-and-drop the .opx file into Origin workspace, a new folder will be created in your Origin user folder. In the folder, there are Microsoft Visual C++ workspace, Sample data and Instructions to show you how to create such DLLs.