Improvements everywhere. Sometimes really great performance boosts, like Array.prototype.sort up to 4x faster, or general 8% speedup with a cache improvement.
How does Code Caching work for Chromium's V8 JavaScript compiler?
Deep dive by Leszek Swirski on how repeat views not only leverage the browser cache for the network response, but also the compiler bytecode. It even includes a limited amount of state snapshot if the initial execution appears deterministic (eg save runtime execution logic for defining and exporting functions).
Good reason to keep library code definitions simple.