Factorio Forums

No save file needed for this, as it's very easy to reproduce:

Use any bulk/stack inserter to drop items into lava, you can see that it takes a very long time (and you can see the stack going down 1 item at a time) to "carefully insert" the items into the lava, instead of just chucking the whole stack. I feel that this is a bug because it doesn't make any sense that it takes longer to chuck garbage into lava than to insert something into a machine where it's going to be used. I've always worked around this on vulcanus by just putting more inserters on a longer belt to offload the stone into lava, but when you need to void something at very high volume it becomes an issue

Machines have an Inventory that holds multiple Items. Tiles can have multiple Items in different spots, but only 1 item per spot. Items are dropped onto the ground 1 at a time, because the Ground does not support Item Stacking. This puts the Inserter into "Target full" state.
If the Target Tile is Lava then the "Item on Ground" is deleted, but the Inserter is still in "Target full" state for 1 Tick. On the next game Tick it will drop another Item, until empty. Good Luck!

Thanks for the report however this is working as intended.