How We Refactored 100k LOC of Python 3.15 to Rust 1.86 and Cut CPU Usage by 55% at a Fintech Startup

At LedgerFlow, a fintech startup processing 12M+ daily transactions for SMBs, we hit a scaling wall in Q3 2024. Our 100k line-of-code (LOC) Python 3.15 codebase, which powered transaction validation, risk scoring, and ledger updates, was consuming 80% of our production CPU capacity during peak hours. With user growth outpacing infrastructure upgrades, our cloud compute costs had risen 42% quarter-over-quarter, and p99 latency for critical transaction endpoints crept above 400ms. We needed a solution that would cut resource usage without sacrificing reliability or developer velocity.…