I think the pressure of “I need to show the world that I AM a top driver now” has gotten to his results. The problem is that n a frontrunning, and probably expected championship contender, the pressure will increase a lot.
While of course the press machines, on Perez’s side and McLaren’s side, have said that they’re expecting Perez to win from the get-go, and to push for the title, IMO it’s not gonna happen. Maybe one or two wins, but not a “regular” win contender in the sense that we see today’s Vettels/Hamiltons/Alonsos are regularly in victory contention for maybe about 90% of the races.
In fact I think Perez will probably get knocked out of championship contention at least 2, and more probably 3, races before Button does (whenever Button gets knocked out – if ever)