TanStack Router, a type-safe routing library for TypeScript, made performance improvements to address a bottleneck in TypeScript's language service when handling large route trees. The issue stemmed from the language service having to infer types for the entire route tree, even when only a specific route was referenced. To solve this, TanStack Router introduced a new approach for file-based routing where the route tree is explicitly declared instead of inferred, allowing the language service to infer types only for the relevant route.