I think you've got a bad memory.
Sterling was having his confidence built up from being public enemy number 1. He had to be taught how to finish, he had to get protected from the media who were crushing him and he was 21.
Sane and Bernardo both came to new countries, Sane didn't speak the language, and both were playing well by early December despite being unfamiliar with the league and being very young themselves.
So Mahrez despite being 28, fluent in English, familiar with the league having won POTY, is 4 months behind those three.
But that doesn't even include how both Sane and Bernardo were at least trying to do what Pep wanted. They were getting game time in every match because while they hadn't fully adapted, they were trying and they were useful. Mahrez sometimes doesn't even get on the bench. He very rarely gets brought on.
From January to March in their first season under Pep -
Sané - 90 minutes in 4/5 League games he was fit for. 360 mins played.
Bernardo - 90 minutes in 3 league games, 35 mins of game time as a substitute in 6 games total. Played every game. 305 mins played.
Mahrez - 0 starts. 0 90 minute games. Didn't get on the pitch for 5 out of 7 games. 24 minutes total game time.
Mahrez's first season is nothing like Sané or Bernardo's. He has none of the excuses they had for starting slowly, and most importantly he's not being treated like a new signing being bedded in by Pep.