Setup overview
Issue
Issue:Setup I were testing this with compresses output belt properly with splitters, but I rearranged it in this release to have inserters and sideloading do compression instead. Rarely but still, there are gaps in output belt - it seems sideloading at last smelter still leaves gaps.
Additionally, each lane needs a bit over 4 smelters to be fully compressed, so they should easily accept input from all 4, making only last 9th smelter that's supposed to fill the gaps back up, yet at each lane 4th smelter gets backed up despite lane not being compressed yet.
Granted, it might just be issue of inserters going 'out of sync' with gaps when smelters are speeded up that much, not actual compression mechanics, but didn't test it throughtly yet. Fast inserters are surely enough when it comes to througtput alone to unload smelters to an empty belt.
Note: This was tested with creative mode mod, both save file and BP string have entities from it, might need to replace with infinite chests or whatnot if tested without mods.
BP string