Java probably outperforms JRuby primarily because the JVM was designed and tuned to run Java.
But in any case, if Twitter's architecture is truly scalable then any intrinsic slowness of the language shouldn't be a big problem, because they can just toss more hardware in to compensate. What is a problem is a buggy VM that leaks memory. To run thousands of instances in a heavily instrumented way, the VM must be stable and predictable.
Statically vs dynamically typed peformance difference is clear for virtually any static or dynamic language you care to name, it has nothing to do with the jvm being "tuned" for a language:
1) Their backend crashes constantly during normal use, so they have to shut it down when nobody is around to babysit.
2) The form submits directly to some poor soul's email, who then has to copy it onto a piece of paper and drop it in a folder. To manage the workload, the form is simply disabled when this person is not around.
3) The job of "web server" has not been mechanized in this particular office, in order to preserve the charm that only comes from a web site lovingly typed in real-time by a hard-working civil servant.
It's YouTube's responsibility to implement a system that doesn't defraud people. The users getting ripped off have no direct relationship with the other users who are abusing the system, and no way to hold them accountable.
YouTube has become a Kafkaesque nightmare, at least for regular users. They recently opened up monetization to everyone, but if you actually try to use it, half your videos will go "under review" and you will be asked to provide proof that you own all the content. They won't tell you specifically what you have to provide, and nobody I know has managed to figure it out or get a video out of this state. Of course, there is no way to contact anyone or get any more information.
Apparently, "full partners" can actually get real customer service and avoid these problems, but you need to get on the order of 1000 views/day before they offer you that, which most people never achieve.
I will absolutely never do business with that company.
Yes, many people see it this way, while many see it as black and white in the other direction: CL doesn't own the content on their site (this is a fact) and thus has no say in how others use it (debatable).
Mix black and white together, and you get... gray!