Writing XLSX files involves two concerns that have nothing to do with each other: OOXML correctness — relationships, content types, namespace declarations, theme references, drawing rels. Get these wrong and Excel may refuse to open the file or report a recovery dialog. The complexity is bounded but the surface is large. Per-cell throughput — <c r="A1" t="s"><v>0</v></c> repeated millions of times. This dominates write time at scale. Yet most libraries solve both via a heavy object model (Apache POI's User Model — correct but slow) or hand-roll everything (fast but you own every OOXML edge case). There's a middle path: let POI handle correctness, let StringBuilder handle the hot path, split them at the format boundary.…