Q: The first time I changed the oil, I felt that 1 was "meat", and 1 was shifted to second gear. Why?

Regarding the problem of "1 gear" after the first oil change, and the problem of 1 gear changing to 2nd gear jitter, I went to the 4S shop to check and said there was no problem. I asked to wash the throttle valve, and I felt the same when I came back. I searched the internet and found that someone was in the same situation as me, but my description was inaccurate. The phenomenon should be: when the cold car starts in the first or second gear, the speed can't go up, but there is a momentum. I wonder if the masters have any brilliant ideas? thank you