The affect that I (think) see is a more even tonality, not a contrast change. But then, one adjusts the contrast as needed for whatever one's methods are.
I tend to keep the printing process/method consistent and then select a scene, expose and develop a camera negative to match the process/method. I make pt/pd prints with no contrast agents, so my negs have a lot of contrast with healthy shadow areas, which means long exposure times. Whatever the longer exposure times has done to the contrast is taken care of automatically by the rest of the process.
The images that used only a 6 minute exposure were much thinner 120 roll film negs not processed originally for alt processes -- using pd and Na2.