Enhance JS Mobile App Script

About 40 % of my JavaScript-based app is already in place. It runs smoothly on mobile and I would like to keep that focus while also ensuring it renders well on desktop browsers. I now need a developer to take the existing codebase, tune the performance, and build two new business-specific features that I will outline once we start.

Here is what I expect:
• Review the current repository, spot bottlenecks, and refactor where it makes the biggest impact on speed and responsiveness.
• Implement the two new functionalities exactly to spec, wiring them cleanly into the present architecture. They are business-centric, not generic modules from a library.
• Make sure the finished build runs without issues on both mobile and desktop. Responsive layout is already partly handled; some tweaks may still be necessary.
• Deliver well-commented code, a short change log, and clear setup instructions so I can continue development after hand-off.

Tech stack so far is vanilla JavaScript with modern ES modules, a lightweight CSS framework, and simple REST calls to the back-end API. If you are comfortable profiling JS, trimming payloads, lazy-loading assets, and writing clean, modular code, let’s get this wrapped up quickly.