[nnq]

no, I didn't skip it, and I can only guess that either they considered they wasted to many resources for such a "small" performance gain or didn't have people with enough skill in the right areas to keep going this direction. But no, I don't know all the details and steps of Twitter's switch and my experience is in the Python ecosystem but I think lots of things are similar...

[Xylakant]

Well, there are several problems with MRI-1.8.x/REE that come into play when you try to go for raw performance. The biggest problem is probably the "stop the world" GIL that occurs around the GC and AFAIK around all external C calls. The second biggest problem is that the lack of multi-core capabilities. Both can bite you pretty badly when you're aiming for raw performance of a single process. They're both not as bad when you're doing web-level stuff where it matters little whether you start one, two or 20 processes to handle your load.