No, but that's because WebKit has other requirements, like a short startup time as well as memory constraints. If WebKit were a long-running server process, a Java version could well be faster (although WebKit's most performance-critical code runs on the GPU anyway, where Java wouldn't have any advantage).
There are other issues as well (maybe I'll write a blog post about them): when it comes to throughput, Java is extremely hard to beat (i.e., it's certainly possible, but not without a lot of work); when it comes to latency, a Java project needs work, too; when it comes to a lot of concurrency, it's almost impossible to beat Java with C++, even with a lot of work (depending on the complexity of the concurrent code).
There are other issues as well (maybe I'll write a blog post about them): when it comes to throughput, Java is extremely hard to beat (i.e., it's certainly possible, but not without a lot of work); when it comes to latency, a Java project needs work, too; when it comes to a lot of concurrency, it's almost impossible to beat Java with C++, even with a lot of work (depending on the complexity of the concurrent code).