Pit stops and their significance have changed since last year when refuelling during the race was banned.  This meant that to gain any strategic advantage in the pits, pit crews had to speed up changing of the tyres.  Last year pit crews were perfecting the art of tyre changing in about two seconds when the FIA stepped in to slow them down in view of safety.  This year thanks to the unpredictable nature of the Pirelli tyres which has been necessitating an increase in pit stops, the focus has once again been on pit crew efficiency.  In the races held so far apparently Mercedes is the fastest and Ferrari is languishing in position seven as per motorsport.com.  The Mercedes pit crew maybe fast, but so far their car has been anything but fast and obviously therefore Mercedes has to work there in order that the work of their pit crew become a thing of consequence.