A setting to disable lightning updates would be useful because it allows both for faster redstone (pistons which move many blocks at once, and repeaters, as long as accurate lightning is not a concern) and more creative building (placing a torch, enabling the setting, and breaking it, leaving a ghost light behind).

Except for the main suggestion mentioned above I have two more suggestions to make, related to light levels:

First is to fix the age-old glitch that appears mainly during world generation and generates some blocks as having a light level of 0 despite appearing like they should be lit up, and updating these blocks causes them to gain their correct light level. I assume this may be hard to implement efficiently, though.

Second is a setblock or fill-type command that sets the light level of a block or cuboid (or more advanced shape, using Scarpet) to some value, which would also help with the development of farms where it is often needed to build large roofs in order to lower the light level to allow for mobs to spawn. It would also allow to perform experiments with light levels and mob spawning, with more ease than was otherwise possible.