Gas is faster to respond, coal, not so much.

From the Goog:

Starting up a coal-fired power station depends heavily on the plant's current temperature, taking anywhere from 2 to 48 hours to reach full operational capacity. Because of massive metal boilers and turbines, the heating process must be slow to prevent severe thermal fatigue and equipment damage. [1, 2] The startup time is broken down by the plant's previous state:

  • Hot Start (less than 8 hours offline): 2 to 4 hours. The boiler and equipment are still warm, allowing for a relatively quick resumption of steam production. 
  • Warm Start (8 to 120 hours offline): 4 to 8 hours. 
  • Cold Start (More than 120 hours offline): 12 to 48 hours. The plant must be heated from room temperature, which involves initially burning expensive natural gas or diesel just to safely warm the furnace and metal pipes before coal can be introduced. [1, 3, 5]

To explore how these heavy thermal operations impact the broader electricity supply, you can review the U.S. Energy Information Administration's grid reliability data or dive deeper into the technical challenges via the Environmental Protection Agency's Coal Startup Report. [6] If you are interested in the broader power market, let me know:

[1] https://www.quora.com/Why-its-not-that-easy-to-start-operati...

[2] https://www.quora.com/How-long-does-it-take-for-a-thermal-po...

[3] https://www.epa.gov/sites/default/files/2015-11/documents/ma...

[4] https://www.quora.com/What-is-the-minimum-time-required-by-s...

[5] https://www.solarquotes.com.au/blog/inflexible-fossil-fuels/

[6] https://www.eia.gov/todayinenergy/detail.php?id=45956