Why would you find this surprising? C is usually less than 10x faster than the next tier of languages; Java, SBCL, GHC, C++, etc. 10x sounds like a lot, but considering languages like Ruby are often 300x slower than C, and they still perform just fine, you can see that Java's "slowness" is almost irrelevant.
Also, git spends a lot of time doing IO -- waiting for your hard drive to spin around is as fast in C as in Java.
(BTW, this is not advocacy for Java, rather, it's advocacy against using C when extremely good languages, like Haskell and Lisp are nearly as fast and are much easier to write.)
For these benchmarks, Java is as fast as gcc. Git is a different workload, so I'm sure JGit is not equally fast as C-git, but it's not going to be unusably slow either. (As time goes on, C is a solution to fewer and fewer problems.)
Shawn is the author of Repo, Gerrit 2 and we use those tools and jgit quite a bit for distributing and developing Android. He's pretty awesome. (works for me, I'm proud to say)
Taking x2 as long as C is a great achievement, especially with Git, which is already extremely fast.
But they've done it by optimizing hell out of Java - it's far from idiomatic java, more like the C-style java you see in the standard Java libraries (but to be fair, C-Git has also had hell optimized out of it).
