Hi Ethan.
Maybe it's the translation but I am a bit confused. Are you saying your actual pit stop is slow compared to your competitors or are you saying your car is slow in the race following your pit stop?
If it's the latter you need to consider your strategy. Your last race was Monaco, 50% race distance. The strategy you used was 5 laps of SS followed by 24 laps on Mediums.
You're Rookie tier so presumably your FE attribute is quite low, how many litres of fuel did you load for 25 laps?
It's not surprising you felt the car was slow when very heavy on Medium compound tyres compared to your first stint of only 5 laps on SS.
I suggest you read this thread which will point you in the right direction...
[2021] iGP Manager complete guide