This is Settings->Inventory Importer.
Just uploaded 350 lines. Took 21 minutes. That’s several seconds each to find the SKU, read a number, add a number to it, and write it back. This used to go faster than I could watch it back when I first started using inventory, but in the past many months it gets slower and slower (it seems).
Please tell me you don’t have a TRS-80 handing this. 
If you do, can we all chip in for an upgrade?