Scalable Next.js Commerce Upgrade

I’m moving the production e-commerce platform from a brittle monolith to a fully-type-safe stack based on Next.js v14, NestJS, PostgreSQL, Auth.js, and Stripe. The immediate, highest-priority task is a thorough refactor of the existing legacy codebase so we can eliminate the high server response times that are slowing checkout and catalogue pages.

Once the worst bottlenecks are gone, the roadmap continues with clean REST/GraphQL APIs and a hardened Stripe integration, all built with TDD and modern best practices in mind. You’ll be working inside a GitHub repo with CI already wired for Jest and ESLint; every merge must keep tests green and Lighthouse scores healthy.

Key context you should know:
• Current pain point: performance inefficiencies, specifically high server response times.
• Tech already in place: Next.js 14 on the front end, NestJS services layer, PostgreSQL 15, Auth.js for sessions, Stripe for payments.
• Priority order: 1) Refactor legacy code, 2) Build clean APIs, 3) Finalise secure payments.

Deliverables (acceptance criteria):
1. Refactored modules pass existing unit tests plus any you add, with p95 API response < 200 ms under 500 concurrent users (k6 script provided).
2. New or updated endpoints documented via OpenAPI and verified with Postman collection.
3. Stripe checkout flow completed end-to-end in test mode, with webhook handling and graceful failure paths.
4. Pull-request walkthrough recorded (loom or similar) so my in-house team can follow architectural decisions.

If you’ve shipped something similar—ideally with Next.js + NestJS—and can point to benchmarks or repo snippets, that will help me shortlist quickly. Feel free to DM links to live demos, PRs, or case studies.