Options depend on other things such as DC impedance.
If C-only, perhaps an op amp buffer with an output series
resistance (in the usual C-driving way) could do the job.
Depends on the step -> slew rate -> current, and "how
close is close enough" (settling to 1/2 LSB maybe, but
how many bits and what signal range?). Maybe a current
feedback style for speed (if you are not being told what
technology you can and can't use).
Now voltage references don't usually do much slewing,
maybe once per power cycle, so what's with the "fast
settling" anyhow?