Mag was relatively in clean air after/on lap 3 onwards! unless he burns a lot of fuel for the first 2/3 laps that compromised half of his race distance, then he was just plain slower than Button at that race. on lap 3, he was already behind Button-Alonso fight. And yes, the last half of the race he was slower all the way through the end! Also the first 14 laps Mag was generally slower than Button. Button was slower from lap 15-20 probably because Alonso wasn't a threat anymore, thus he was slowing his pace. Although I say he was slowing his pace, it's more that Button run at a consistent pace since Alonso wasn't really a threat anymore while Mag trying to catch Alonso.
Either Mag was under fueled, or he used too much fuel for that 2-3 laps that compromised half of his race distance (impossible), or he used too much fuel for the first stint. All this scenario still puts him slower to Button at that race either by suggesting that Button carried more fuel but still faster than Mag, or Button drive more economically than Mag while still being faster while both were in clean air. So I don't see this speed that you're talking about. Yes, Mag was faster in quali on the first half of the season, but the 2nd half (or maybe the last races), either Button got faster or Mag got slower. When both finished the race, Button was 14 times ahead vs 3 for Mag!
And Button was always fast. Not the fastest but fast, even in his rookie year. He was entering F1 with less experience than Mag or even Max Verstappen! Definitely not as groomed as Mag or the current crop of young drivers. Although he was beaten by Ralf, he was still wanted by some teams. So I don't know how long the time Button's took to find his ultimate speed, but he was definitely fast enough to be wanted. I just didn't feel that Mag will become something special. Actually, I feel that Vandoorne could be the better McLaren young driver. Mag was already in formula racing since 2008 while Vandoorne started in 2010 and in his shorter career, his results are more impressive than Mag. For comparison, Button got 2 years of lower formula experience before coming to F1. While it suggest that Button got more experience than Max Verstappen as a rookie, he races less vs Max and his racing experience mainly in Britain vs a wider range of track for Max. Out of the current F1 drivers, probably only Raikkonen and Alonso that entered F1 with less racing under their belt.
So as you can see, I'm really rooting for Button to be in F1. It isn't because Mag isn't good enough to be in F1, but because Button is simply the better driver for right now and I definitely want to see Alonso vs Button! Some of the better wheel to wheel racing this year are when Alonso and Button met together on race tracks. It might become Alonso vs Kimi 2.0, but I still want to see it.