Worked on the vuejs/core repository to optimize reactive system size retrieval, focusing on improving performance for large datasets. Addressed inefficiencies in the reactivity instrumentation by replacing Reflect.get with direct access to the size property on target objects, reducing per-lookup overhead and enhancing UI responsiveness. This change, implemented in TypeScript, maintained the existing public API while delivering runtime efficiency improvements across reactive workloads. Demonstrated core development and performance optimization skills by ensuring the update introduced no new critical bugs. The work was tracked with a traceable commit, reflecting a methodical approach to instrumentation-focused engineering within the Vue.js core codebase.