A mixer can run at the full speed of its input belts (ie given two full belts input, it will output two full belts). In the naive solution you are effectively

A mixer can run at the full speed of its input belts (ie given two full belts input, it will output two full belts). In the naive solution you are effectively allocating a single belt output from each layer-1 mixer, so the system will run at half throughput. But the fan-out to a pair of mixers in layer-3 does mean that your 4 output belts are balanced, which might be good enough in some cases - for example of downstream smelters have lower capacity than the theoretical 4-belt max throughput.