Because with 80%+ unused code, your instruction cache is gonna choke. Also JITs can remove many more type checks, because it knows when a check would be redund

Because with 80%+ unused code, your instruction cache is gonna choke. Also JITs can remove many more type checks, because it knows when a check would be redundant. See e.g. Lazy Basic Block Versioning[1], but existing tracing JITs do quite well also. 1. http://arxiv.org/abs/1411.0352 http://arxiv.org/abs/1411.0352