To answer OP's question, Metro Atlanta vote tends to report before the rural "downstate" vote. This is in contrast with northern states like Wisconsin and Pennsylvania, where rural votes are counted relatively quickly and very Democratic inner-city precincts take the longest to report. With Clinton performing very well in Metro Atlanta this year, more raw votes were needed out of South Georgia to show that she wasn't going to win the state.
The rural areas of South Georgia are notoriously slow-voting, and the large numbers of counties (159, second only to Texas) also means that it takes longer for votes to be counted.
This isn't necessarily true. You are right about the broader premise - that the metro was substantially more Democratic this time around, and that led to the delay in the call - but the way in which you've said Georgia tends to report is not the case.
The distribution of counted votes within the first 90 minutes of so of the polls closing are disproportionately from rural Georgia, and of that, disproportionately from South Georgia - it's a small area in terms of population so it's never a huge chunk once votes really start coming in, but South GA is almost always over-represented in the first couple of hours of counting.
Like I said above, ATL proper is on a one-hour delay, so your most Democratic 5% of the state doesn't begin trickling in until an hour after the polls close at minimum, and are always among the last precincts to fully report. Fulton, Dekalb and usually one other urban county are always the last three to finish counting. The heavily Democratic and populated counties always take the longest to fully report. The metro/suburban areas aren't necessarily quick nor slow to report - and can vary considerably in terms of when each individual one wraps up the count - but they are usually not among the first.
This is why if you watch GA's GOP margins from 7 PM on, it consistently shrinks as the night progresses (and usually continues to do so for days afterward). Almost always, the GOP is leading in GA by anywhere from 15 to 25 points for the first 60-90 minutes (because it's South GA/rural GA reporting), and then it begins to creep downward rapidly (partial returns from urban counties), slows a bit (partial/full returns from suburban counties), and then continues to dwindle slowly over the remainder of the evening (remaining returns from urban/suburban counties).
By and large/in the aggregate, Georgia reports in a way comparable to population distribution: smaller and more rural counties report first, suburban counties follow, and then most populated/urban counties are last to finish.
In fact, I believe if HRC had matched Obama's 2012 performance in Non-Atlanta Georgia, Trump would have won the state with <1%. Had she matched his 2008 performance, she would have won it.
If you take these boundaries - the metro from 2016, the rest of the state from 2012, and use the statewide third-party vote from 2016 - then Trump
still wins by 125,000 votes.
Trump 49.4% 2,014,778Clinton 46.3% 1,889,561Other 4.3% 174,378
