Developer halves checkout API latency to under 900ms using AI-assisted debugging

A software developer reduced their checkout API's p99 latency from 2.1 seconds to under 900ms in a single afternoon by using Claude Code as a debugging partner. The performance issue had gone unnoticed for about a month until a support ticket flagged that checkout felt occasionally slow. By feeding real trace data to the AI tool, the developer discovered that orders with more than 12 line items were consistently the slowest, pointing to a non-linear scaling problem. The root cause turned out to be an O(n²) bug in the discount-application logic, where every line item triggered a full rescan of all items to check for bundle discounts. An earlier attempt to fix the issue by adding a database index had failed to improve p99 latency, underscoring that tail-latency problems often require targeted fixes rather than average-case optimisations.