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More likely a tyre issue, Leclerc started on the soft tyres which provide more grip initially but quickly fall away (in fact, most teams didn't plan to run them at all due to their short lifespan. I think the only reason anyone used them was due to thinking that they'd come up to temperature quicker on the drying track). I definitely recall Leclerc being quite vocal about how slow he was on his worn soft tyres, but they thought it was going to rain soon after, so didn't want to pit only to have to pit again a few laps later for wet tyres.
Power output should be consistent across the race, I think they use each engine for around 6 races or so these days. The only thing that really affects the power output of the engines is altitude as a result of lower air density (though this also affects the drag significantly, I think the highest speeds of the season are in Mexico which is over 2km above sea level)
Slow course with very little in the way of straight track means their engine's power deficit isn't as obvious. Still lapped by the Mercedes though so nothing to shout about yet.