Running events fundamentally involves more work than running pure JavaScript code. For example running a list of actions involves a loop, and if you write JavaScript code that just does a series of tasks without a loop, then it has less overhead.
In many cases the overhead of the event engine doesn't matter though, and as I mentioned many aspects of the engine are extremely well optimized and would be difficult to match in your own JavaScript code, e.g. pathfinding.
If you want to know the exact performance difference, as I said, you can get a very precise answer by measuring it yourself.