Fcn block slowing down compilation

In my Matlab 2017a Simulink model I recognize a massive brake during code generation of a subsystem (during creation of the subsystem's C mex S-Function). The Subsystem itself includes (among other things) a Fcn block (not Matlab Fcn, not Embedded Matlab Function, not Level-2 M-file S-Function, not S-Function). This one includes a huge function with more than 7500 characters... Simulink of Matlab 2010a compiled the subsystem very quickly, independently of the number of charakters in the Fcn block, whereas Simulink of Matlab 2017a hasn't finished the build process after 19 hours... Has anything changed in those seven years concerning the maximum length of characters in the Fcn block? Any answer or idea would be appreciated.