ClearFunds: rebuilding a fintech platform from Rails monolith to Next.js

ClearFunds had a 6-year-old Rails application that was slowing down development velocity and underperforming on Core Web Vitals. Page load times on mobile averaged 5.2 seconds — below the threshold that Google considers acceptable for search ranking.

Simplileap conducted a 3-day technical discovery to document the existing data model, API dependencies, and critical user flows. The architecture decision: Next.js App Router frontend with a FastAPI Python backend, migrating gradually rather than a big-bang rewrite.

We implemented a strangler-fig pattern — new Next.js pages routing alongside the Rails monolith during the transition. Critical financial calculation logic was extracted to the Python API layer with full test coverage before the frontend migration touched it.

Results after 14 weeks: Lighthouse performance score improved from 34 to 89 on mobile. Time-to-first-byte reduced from 780ms to 310ms. Core Web Vitals entered the 'Good' band for LCP and CLS. Development velocity increased — new features that previously took 3 weeks now take 1.

The CTO noted that the quality of the codebase and documentation was 'what I would have wanted from day one'. The investment in types, tests, and architecture decisions upfront paid compound returns.