No, it doesn't improve. Normal stops are 4.3-4.7 seconds, good ones about 4.0s-4.2s and bad ones 4.8s-5.0s. For all I know it's just random how good your stop will be.
For example here the winning driver from our Valencia race, given that link works for everyone. 3rd stop is 4.7s, 4th is 4.0s and 5th again 4.8s. Same team, same driver, same amount of laps in the same race and still that 0.8s difference.
https://igpmanager.com/app/d=resultDetail&id=106698273