I have 2 zones in my house. I just installed yesterday two of nest learning thermostat ( one in each zone). One of them working properly and got the required temperature in very short time but the other one takes a very long time ( more than 10 hours to got it) ? By the way the old thermostat was got the temperature very fast either.
Sorry for the delayed response but thanks for reaching out to the Google Nest Community.
Do you still need help with this? If yes, could you provide a rundown of the steps that you've already tried? Also, if possible, could you swap the working one to this other one in question then observe if the delay persists?