TL;DR This is about pure Ruby code optimized by JIT vs manual optimization by writing C extensions (native code). There is Ruby code around those extensions bu

TL;DR This is about pure Ruby code optimized by JIT vs manual optimization by writing C extensions (native code). There is Ruby code around those extensions but > The JIT cannot know what the native code will do to parameters (possibly mutate them), it cannot know what type of Ruby object will be returned, if the native code will look at the stack, or if it will raise an exception, etc. Thus the JIT is forced to have a conservative approach when dealing with native code, essentially forgetting much of what it had learned, and falling back to the most conservative implementation possible. Hence the JIT often makes pure Ruby code run faster than the code with C extensions. I think I heard the same argument many times with other JITed languages, interpreted or compiled.