The main disadvantage is strong coupling. The shared state inside the static makes it more difficult to test your code. There are ways to reduce this coupling, the most obvious being that you push as much logic as possible into functions that accept a reference rather than use your static directly. Then you minimize the size of the code which is difficult to test, and cover it with end to end testing.

The OnceLock does impose 1 atomic operation of overhead, whereas an Arc imposes 2 (in this case, it can be more if weak references are used). However neither are likely to be important when you consider the overhead of the Mutex; the Mutex is likely to involve system calls and so will dominate the overhead. (If there's little to no contention then the Mutex is a lot like an Arc.)

Your alternative would be to use `Box::leak()` as the parent comment describes, which would force you to pass a reference around (resulting in better coupling) and eliminate overhead from OnceLock. However, it's unlikely to result in any material performance benefit, and you could reduce coupling without departing from your current approach, so I don't think either approach is clearly or overwhelmingly better.