You also don't want to be too fast, because of stray inductance between the regulator output/feedback terminal and the load being measured.
There's example of this in the blackdog thread,
https://www.eevblog.com/forum/beginners/how-does-blackdog_s-psu-work where the measured waveform is spikey when measured at even a slight distance, because the measurement is very sensitive.
On a recent project, I created my own ad-hoc standard for output impedance, for a number of reasons:
1. The power supply output impedance was a critical aspect of the design;
2. The output was well filtered, and filters have a characteristic impedance;
3. The load will be connected through some distance regardless, so some allowance must be made, on the load, to account for the source impedance;
4. The initial project was powered by hookup cables of modest length, so we knew there was a maximum allowable inductance of at least as much.
The way I tested to the standard, was to use the load-step circuit (idle load resistor || switched load resistor), and RC-filter the output voltage, at a given -3dB point, to obtain the measurement. The filtering ignores high frequency properties of the internal filter structure, and allows me to approximate it as an RLC network.
Finally, I solved the RLC values, by fitting a curve to the waveform. This gave reasonable values for inductance, when the supply compensation was reasonable. (When compensation was not reasonable, it stood out.)
A rather lengthier process than done for most supplies, but every step of the way is perfectly reasonable, and the data are good and useful.
Tim