[cprobertson1]

Both MS and ES have a traveling strip of many rows of pixels that are being simultaneously exposed.
...
The reason for the difference is two-fold. First the only ways to stop an exposure of a pixel is to either cover it with a mechanical shutter or read the pixels data out of the sensor. (Each pixel is like a tiny solar panel that cannot be "turned off" -- if light is hitting the pixel, a charge is building up and changing the pixel value.) Second, it takes a fair amount of time to read all the pixel signal data out a large sensor. For MS, the shutter quickly exposes and then covers the rows of pixels and then read out process happens slowly behind the closed shutter. For ES, exposing each row of pixels can't go any faster than the read-out process so the ES exposure process is much slower than it is with MS.

Ah-ha! Many thanks! That definitely clears up the reason behind it! I was unsure the mechanism behind mechanical shutter (apparently) being able to read the sensor quicker than ES - many thanks!

[MrB1]

A few points about using the KP's electronic shutter:

As SR is disabled with ES, bear in mind the rule of thumb for reducing camera shake motion blur:- minimum shutter speed should be no slower than 1/(FF equivalent focal length) seconds.

ES removes most of the repeated mechanical clatter of continuous or bracketed shooting, and reduces stress and wear of the moving parts.

Every ES shot adds one to the KP's shutter count. So, in the future when you might want to sell or trade the KP, it might show a high shutter count despite the mechanical shutter having had hardly any use.

Philip