Found in the comments of an "elektor"-article:
RS-elc vor 1 Monat
Arduino IDE version compiler effects
After some speed limit testing I found that the code for this project runs about 20% more efficient when compiled with the Arduino IDE 1.04 ( that was used) compared to the same code compiled with the newer IDE version 1.8.12.
So the newer IDE produces slower code here, resulting in more processor load.