How I Improved API Throughput by 326 and Reduced Latency by 99.7% in a .NET Backend

When I seeded my backend with 600,000 car records and 1,000,000 posts to simulate real traffic, my API nearly fell over. 54% of requests were failing. The p95 latency on the car listing endpoint was 30 seconds . Throughput was sitting at a painful 2 req/s . Eight weeks later: 807 req/s , 79ms p95 latency , 0.26% error rate . This is the story of how I got there. What I Was Building GearUp is a Facebook Marketplace-style platform for cars. The feature set is fairly complex: KYC verification flow