[apendleton]

Actually, my experience with PyPy, while generally positive, has exhibited many of the characteristics that article talks about in terms of downsides to "sufficiently-smart compilers." It's almost always much faster than cpython, but how much faster is highly variable, and not especially predictably so; seemingly-insignificant changes can have large and unexpected performance implications, and it's difficult as a developer to have the proper mental model to figure out what's actually going on.

In CPython land, Python is slower, but performs predictably, and if you want to speed it up you write C, which is much faster, and also performs predictably, though it takes some developer effort. In PyPy, you get some of the speed of C without the effort, but without the predictability either.

[jrockway]

C is also unpredictable: see the article from a couple days ago entitled "Why does my function run faster when the input array is sorted?"

(The answer was: branch prediction.)

[to3m]

I'm not sure that counts in the same way. You would have the branch prediction issue even if you program in assembly language and have precise control over the instructions the computer executes.