Optimize React GST Invoice App

I’ve already shipped a full-stack, GST-compliant invoice generator built with React (Vite), Firebase Firestore/Auth, and vanilla-CSS components. All major modules—authentication, dashboard analytics, invoice editor with multiple templates, customer and item catalogs, GST reports (B2B/B2C/HSN → Excel/PDF export), public invoice sharing via QR, offline detection, and settings—are live and stable.

Now I want to squeeze every millisecond out of the experience and remove any friction in data throughput. The priority is performance and optimization, with two clear targets:

1. Loading speed: initial bundle size, route-level code splitting, caching strategy, and perceived render time.
2. Data handling: batched/streamed Firestore reads-writes, memoization, state management hygiene, and eventual-consistency quirks that appear under heavy invoice traffic.

I’m open to fresh ideas and tools—whether that’s React-Query/TanStack Query, SWR, Suspense, Vite plugins, Lighthouse-driven audits, Firebase indexing tweaks, or any modern profiling utilities—provided they integrate cleanly with the existing React + Firebase stack.

Deliverables
• A performance-profiled codebase with actionable before/after metrics (Lighthouse, Web Vitals, Firestore read/write counts).
• Refactored code or modular add-ons implementing the agreed optimizations.
• A brief technical report outlining changes, tools introduced, and next steps for maintenance.

Acceptance Criteria
• Core pages load in