>So. Yes, its practical to build Git in a higher level language, but you just can't get the same performance, or tight memory utilization, that C Git gets. That's what that higher level language abstraction costs you. But, JGit performs reasonably well; well enough that we use internally at Google as a git server.
I think that this is the key takeaway for the entire post.
One of the reasons I generally dislike any of the "X IS BETTER THAN Y" bakeoffs is that performance is now so implementation dependent that these comparisons are pretty much moot. Given that basically any non-trivial implementation can be improved, it's difficult to say that anything is faster, especially when one considers developer skill.
Developers should not be chasing the abstract, absolute best performance. Instead, the language used should be the one that delivers performance that is good enough for their client's needs. If they can get it with something that we're familiar with, that's great. If they need to learn a new tool, that's also good. But it doesn't make much sense to throw away all the knowledge that a developer has about a certain language to chase "better performance" with a different one. Most likely, the first effort implementations on a new language won't be nearly as good as the implementations on the more familiar language.
It's generally true that optimized Java won't ever be as fast as optimized C. But for the vast majority of cases, it doesn't need to. Java's speed is enough for those cases. And in the small minority where it's not sufficient, C is still around.