Hi Niels,
The AD5420 has the same settling time specs as the AD5422. The only difference is that it is Iout only, rather than Vout/Iout.
In terms of step size vs settling time, looking at the "4 mA to 20 mA Outout Current Step" plot (Figure 36) in the AD5420 datasheet - this shows a 16mA step, the slewing portion of which takes approximately 6us. Doing some calculations on the linear portion of that slew, we get a slew rate of approx 3mA/us (~12,288 codes/us). Now, this is the slewing portion only. Given a typical settling time spec of 10us for a 16mA step (to 0.1% FSR), we can allocate approximately 3-4us for the linear settling portion. So, based on your code step requirement, you would allocate 3mA/us slew rate for the slewing portion and ~3-4us for the linear settling portion.
Re the max update rate: (24 x 33ns) + 5us = 5.8us => ~170kHz. This will depend on the step-size excursions you are doing on each write to ensure the output is settled before the next write command is executed.
Hope this helps,
Tracey...