Wolli is the new expert for Serbian traffic. A veteran expert on Greek food. Well, any food.
My son and I returned yesterday (Jul 28). The traffic southward was lively, but nowhere was it at a standstill. My impression was that the wait at border crossings was also reasonable.
Remember not to go around Belgrade (the ringroad is not what you want to do), but follow the highway to the city and drive straight through - it's all highway, no lights no staus and much faster than the still incomplete roundabout.
Messy points: toll (Maut) ramps are again the place where one may have to wait a long time.
There is a section under reconstruction on the Bg-Nis highway, just before Pozarevac and there is massive construction and detouring between Vladicin Han and Vranje.
It's a Landstrasse and the no-overtake detour is very long. Slow traffic, boring, but again, I haven't seen it stand still.