There was nothing wrong with the Merc race pace. They were running the first stint with a one-stopper in mind. NR stayed with this far-far too long, judging by his post-race comments, He has been too stubborn to dump the strategy, while MS timed his pit stop perfectly for a strategy swap. From than on, Both Merc drivers were consistently fast in free air, NR, in fact, was consistently fastest, and MS could stay on pace after he battled through quiet a few cars, so tyre management has took a great step forward too, as already indicated by the team. Sorry, but your lack of insight is slightly disturbing.
A temporary road course is always the worst place to try a one-stop strategy. A safety car ruins everything.