CPU cores have a temperature near 80 °C and then the fan starts to work. Is it too late?
No, it is not a "bad" temperature. If your MBP 13" 2015 model is configured with the default CPU, it is specced to work at up to 105&deg; C. So running at 70 or 80 degrees is no problem at all.

In general you should never change the configuration of your fans away from the defaults supplied by Apple.