I once went the other way (do note, that this was in early 2008, so a lot of things are probably different now)... But for me, it took one day or less from Belawan to Lake Toba, then two days to Bukittinggi (that stretch was quite bad, especially some huge hills, where the road was so steep, that they used Caterpillars to pull buses & trucks up the hill, also did not help, that the road surface had been washed away by recent rains, so it was mostly just rocks and sand... from Bukittinggi, only one (very long!) day to Lubuklinggau, and from there, two days of riding to Bakauheni to catch the ferry to Java. So, about six days of riding for me, althgough I did stay longer especially at Lake Toba and Bukittinggi.
Note, that this route went mostly through some hills or mountains, actually quite a bit of curvy (but bumpy) tarmac roads, too much traffic was not often the issue, but still the road was not very fast. For some reason, especially the braking areas before the bends were often bumpy as hell, that was frustrating, and trees were often casting shadows there as well, sometimes very hard to see the bumps in time. Parts of the road on hillsides gone missing after rains was also something to look out for. As well as bus and truck drivers, who can overtake just about anywhere, and not care about anyone´s security. On a bike, it was better to escape from them in the curvy sections, but when the road straightened, I did not want to go as fast as some of them did, so just pulled over a bit, and let them past there.
You could probably find a more northern route, that would go on the plains... but I don´t know, if that would be faster or not, I´m guessing it depends on how much traffic there is. It would probably be a LOT hotter, though – in fact the temperatures & humidity on the route, that I took, were quite pleasant and mostly because of the altitude.
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