[twotwotwo]

Spelled out, TurboFan and IonMonkey are JITs that take longer to run but can do more optimizations. The idea is to run them on code that executes a _lot_ of times, so the faster code generated pays for the extra JIT time. If you wrote a game in straight JS and it contains some code that runs for a bunch of in-game objects every frame, that's the kind of thing these JITs could really help with.