Nuclear plants are OK at this. The current fleet in North America can ramp at about 5% per minute from 50% to 100%. (Between 50% and less gets very slow). There's also concern that repeated ramping could greatly increase the amount of maintenance required.

So, on the same order as a "fast" combined cycle plant and quicker than a older combined cycle ng plant. But.... pretty slow compared to the 4 minutes from 0 to 20% and 4 minutes from 20% to 100% for an open cycle gas turbine, and a fair degree of ability to react to "step" loads.

So, if nuclear needed to, it could help out in grid stability operations, but you really need something very fast like hydro or open cycle NG plants (or batteries) to do most of the work.