Edit: It seems that it was not caused by this mod, it was low when i load with out it, and high if i load with it, but after i removed it and played a while the GUI update time is high again.
Well yesterday i had an GUI-update-time of 15ms, which is usually <1.
I play with plenty of mods (40) mostly LTN/SE and some QoL, i disabled all mods and enabled them again a few every restart to see which one is causing the 15ms.
It was trainsaver, tho i don't have much further information, all i can say is:
Worked fine before and it does not happen on other maps than my Megabase.
Note: The trainsaver mode was not active during the high GUI update time.