Hallo Rob,

edge speed also has an influence on digital ground return currents which tend to erode ground potential. It has to do with charging speed of stray capacitance and input capacitance of gate inputs. This can be shown by relation C = Q / U. If you take the time derivative, then you get:

I = dQ / dt = C x dU / dt

If 'dU / dt' is the slew rate of digital signal and 'C' is charged capacitance, then 'I' is the charging current, which lasts for the duration of transition of digital pulse. So, when slew rate of digital signal is increased, then the time duration of charging 'C' is decreased, while at the same time the current spike 'I' is increased. And as this current spike will run across digital ground back to the source (to the output that toggled), you will notice an increased ground noise level.

Regards,
Kai