Your React dashboard freezes for 4 seconds every time a user uploads a CSV.

Not a network freeze. A browser freeze. Tab unresponsive. Animations stuck. The "Page Unresponsive" dialog popping up on Chrome.

Here's what's happening in prod:

• User drops a 80MB CSV (200k rows, 40 columns) into the upload zone
• Your code: Papa.parse(file, { complete: ... }) on the main thread
• Parsing + validation + schema mapping = 3.8s of synchronous JS
• During those 3.8s: scroll dies, the loading spinner stops spinning, input lag goes infinite
• Your INP score on the perf dashboard is now red. Lighthouse is screaming.

Product wants this fixed before the enterprise demo on Friday. Four options on the table:

A) requestIdleCallback + time-slicing — chunk the parse into 5ms slices, yield between rows, let the browser breathe.
B) Web Worker — move the entire parse + validation pipeline into worker.js, communicate via postMessage, main thread stays free.
C) SharedArrayBuffer + Atomics — share the file buffer between main thread and 4 workers, parallelize the parse across cores, zero-copy.
D) Compile the parser to WebAssembly — drop the JS parser, run a Rust CSV parser through WASM for near-native speed.

All four ship in real frontend codebases. Three of them are the wrong pick for this specific problem. One of them is the boring, correct answer that Figma, Google Sheets, and Excalidraw actually run.

One is the senior-engineer trap. Sounds sophisticated in design review. Doubles your infra complexity. Requires you to set COOP/COEP headers and break half your third-party embeds. Solves a problem you don't have.

Pick one — A, B, C, or D — and tell me why. Full breakdown drops in the comments (including which option is the trap, and why the "faster parser" answer doesn't fix anything here).

Drop your answer 👇