blueprint
Output:At crafting speed 1, a smelter with 20% productivity should produce x = (1 / 3.5) * 1.2 * 60 = 20.57 plates per minute.
Current setup has 11 at speed 9.4 and 2 at speed 7.4, giving (11 * 9.4 * x + 2 * 7.4 * x) = 2431.54 plates per minute, which is close to the maximum throughput of a blue belt at 2410. As far as I can see the is no real bottleneck. I'm not quite sure the balancer at the top is needed, but it might help to balance out temporary imbalances. I'm also not sure the bottom splitter is needed, but it divides the output over both lanes so each lane has 6.5 smelters.
Do you notice the stuttering on the input side when the inserters drop their stuff? That's how it works. Interestingly you can see some copper plates flicker on the upper underground belt entrance in the first few frames of the gif. Never seen that in-game though, must have to do with the gif recorder. But it explains whats happening technically: The underground belts just remove the items in front of them when it's time. In fact they are teleporters. The tiny 1-tick gaps in the input mean that there is no plate immediately following so for a very brief period of time an item-sized free slot appears on the belt.
