[0.16.16] Reduced belt throughput when adding inserters

[SIGSEGV]

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I've found some arrangements where adding stack inserters that remove items from a belt between a source and a sink (in this example, infinity chests with loaders on both ends) somehow reduces overall throughput, even though there are still large, visible gaps upstream of where the inserters were added.

The belt patterns are based on balancers from the wiki, though I didn't see the throughput issues without the additional splitters downstream of the balancers. The cluster of combinators in each setup counts items as they pass on the belts, resetting every 10s (credit to XNight: viewtopic.php?f=8&t=29679&start=0#p188125).

The NW and SW setups are identical, except that the SW has stack inserters removing items from the two ends of each underground belt. The NE and SE setups share the same relationship. The N setups consistently achieve belt capacity (80/s for NW, with 2 express belts; 40/s for NE, with 1). However, the S setups have lower throughput: SW hovers around 77/s, while SE is about 39.7/s. There's no visible sign of the belts being backed up just upstream of the splitters (particularly on SW, between the westernmost two columns of splitters), but while it's running you can see stutters in the initial, packed part of the belt.

Anecdotally I've noticed that inserters seem to have more trouble picking up items from underground belts like this in the 0.16 series than in 0.15, but I don't know whether or not that was an intended change.

Save attached.

[kovarex]

This is before the item compression changes in 0.16.25, so I consider it fixed or invalid.