Bulk order CSV, what’s the realistic performance with 5,000+ line items?

5,000 lines is the high end of normal; 12,000 lines is doable with care; 50,000 lines needs a bulk-order asynchronous queue.

• 0-500 lines: synchronous. Submit → validate → cart → checkout in <5 seconds. No special handling needed.
• 500-5,000 lines: still synchronous but with a progress bar. Validation runs in chunks of 200 SKUs at a time to avoid PHP timeout. Total time: 10-30 seconds. Stock + price checks happen before cart insertion.
• 5,000-12,000 lines: asynchronous. CSV uploads → queue job → user gets an email with a "review your prepared cart" link in 1-3 minutes. SKU validation, price-snapshot, and stock-availability happen in the background. Buyer reviews + submits.
• 12,000+ lines: custom queue + multi-batch order (split into 2-3 separate orders to keep the order_items table sane). Often combined with multi-warehouse splitting.

Validation pre-submit: always show buyers (a) which SKUs aren’t found, (b) which are out of stock, (c) which have price differences from their list. Letting them edit / remove problem rows pre-submit cuts post-order disputes 80%.

Anti-pattern: never run a 5,000-line CSV through Magento’s standard add-to-cart events, you’ll trigger the catalog-rule indexer 5,000 times. Use direct quote-item insertion bypassing event observers.