We don't always slow down in the second half. That's ignoring the circumstances of the matches. In the majority of games we've done well enough in the first half to play a different kind of game in the second half. Against, say, Arsenal, when we still needed to push in the second half, we upped the pace until we'd got the goals.
It's always good to score early. That goes without saying, really. If you do that then of course you manage the game more effectively in terms of pace and intensity. I don't think you can make too many judgements as to our stamina when we haven't NEEDED to stretch it so far. Not needing to stretch it, and actively conserving it, is not a sign of its absence.
What we do need to do, obviously, is to defend more consistently and effectively.