Try to use parallel processing in BRep cutting algorithm.

pcbnew/exporters/step/step_pcb_model.cpp

@@ -1022,6 +1022,7 @@ bool STEP_PCB_MODEL::CreatePCB( SHAPE_POLY_SET& aOutline, VECTOR2D aOrigin )
 
             // This helps cutting circular holes in zones where a hole is already cut in Clipper
             cut.SetFuzzyValue( 0.0005 );
+            cut.SetRunParallel( true );
             cut.SetToFillHistory( false );
 
             cut.SetArguments( cutArgs );