I can confirm your measurements. It has nothing to do with the Python-side of things. It is the display and I already suggested to Pimoroni to update the shop page. The 12s is from the datasheet and as always this figure will certainly be valid for a very specific use case. The datasheet is not public so it is hard to tell.
The core update within the screen is about 18s, but with the coding you need to setup things, you are in the range you observed. See also my post here: Inky-Impression 7.3 (2025)