So beside snickering at the "perception" comment. I will try to answer this seriously.
I do think it is interesting to look at the four driver who are not... Rookies or are re-entering F1 after a break (remove Alonso, Tsunodo, both Haas drivers) and look to see how well..
1. The came up to speed in their new car/team
2. How they have been fairing against their teammate.
As I wanted to do something quick and to not delve deeply into per race "why", I am ignoring situations in which one driver or the other either DNF or DNS and I am only looking at finishing position.
Perez (Aston Martin to Red Bull) vs. Verstappen
Avg Delta: 3.86 positions (advantage Verstappen)
STD of per race delta: 2.79
If you were to exclude his second race, his numbers would be much better. Overall, while Max is beating him, Perez has gotten up to speed pretty quickly. Perez decision seems to have been the right thing for RBR.
Sainz (MeLaren to Ferrari) vs. Leclerc
Avg Delta: 1.38 positions (advantage Leclerc)
STD of per race delta: 3.40
Sainz is generally up to speed pretty quickly at Ferrari. Great decision for Ferrari.
Vettel (Ferrari to Aston Martin) vs. Stroll
Avg Delta: 2 positions (advantage Stroll)
STD of per race delta: 3.42
Signs that Vettel is getting his feet under himself, but a VERY slow start for him at the team. Stroll seems to be no slouch. Need more time to see who will get the upper hand
Ricciardo (Renault to McLaren) vs. Norris
Avg Delta: 3.75 positions (advantage Norris)
STD of per race delta: 3.54
I think many eyes were on McLaren to see what the relative performance of these two would be. Starts/Podiums/Win stats for each are...
Norris: 46/3/0
Ricciardo: 196/31/7
While you can argue that Vettel is in the twilight of his F1 career, I think expectations for Ricciardo vs Norris was high. On paper, Ricciardo has it all over Norris. But Ricciardo has generated a few head scratching results. He clearly is in some type of funk. My guess it's a combo of the car not fitting his style and Norris' early results has gotten into his head. Plus, Given Norris is still new and seem to be pretty good. You would expect him to improve. So Norris is probably the real deal as well.
All four drivers here who switched teams have had to adjust. Some better than. To me (my perception!) is that Ricciardo and Vettel have struggled the most. For Ricciardo its particularly damning that the car is clearly quick and he is up against a less experienced driver.
My analysis is VERY quick, dirty and no doubt has some holes in it, but I see some correlation between this and Schummy's deeper analysis
https://tentenths.com/forum/showthread.php?t=156268
Richard