While you were preparing that 
, I entertained myself by calculating the “effective resistance” of a generic small-signal diode, as a function of the voltage across the diode. I-V plots are common, but I couldn’t find a corresponding R=V/I plot so made one myself by stepping through a simulation:
The red line is the cut-off frequency for the equivalent 10μF highpass filter (the properties of which keeps changing during the cycle, obviously).
Zooming in:
EDIT: Turns out the default diode I used has a relatively high reverse current and a lower forward voltage than your typical small signal diode (e.g. 1N914/1N4148) so should probably redo this some day to fix the reverse slope and knee voltage, but the general idea still applies.