Anything that has to be interpreted or un-JIT'd for the most part. Runtime eval, runtime struct memory layout redefining, runtime type change, etc. Basically it

Anything that has to be interpreted or un-JIT'd for the most part. Runtime eval, runtime struct memory layout redefining, runtime type change, etc. Basically it's a trade off between pre-compiled immutable omniscience and developer friendliness. Targeting the former often increases perf due to the assumptions you can make, targeting the latter increases comfort and adoption and usability and simplicity.