Yeah, there's no way they're pushing the cars just yet. Like Colwill said a 3sec improvement in a day just shows they're slowly running the engines/chassis in. With such a huge design change in front/rear wing and nose section the amount of aero data they need to collect is probably one of the number 1 aspects, then you've got cooling and air flow to look at for the engine, baring in mind that at least the engines will have had the advantage of being bench tested LOTS over the last couple of months.