They don't spend that long on it because they have heaps to go through and they get paid for number of papers marked, not how long it takes them.
It's in their best interest to read *some* of it so that they end up giving roughly the same mark as the other person who marks it

At the english language revision lectures I went to one of them was an assessor and she said she spent on average 4 minutes marking the essay part and I think that is being generous tbh

In any case, yeah make the start of your essay good, as opposed to starting off crap and making a comeback at the end, because they're only human and first impressions count.
They usually can guess what they'll end up giving an essay based on the introduction alone.