The OP made too general of a statement because there are certain cases where V8 can preform better than the JVM; however, generally Java does seem to perform better (ignoring startup time).
Heh. Last time I checked v8 regexes were really fast, except their string builder was a disaster... it wrapped every piece of string as a js object (gc and everything).
Sounds familiar to Java's own string concatenation. In Java you use StringBuilder / Buffer to create larger strings from smaller ones; in Node you'd use Buffers, although I never dived too deep into those.
That is far from clear. I've been using nodejs for some computational work using https://github.com/substack/gamma.js and https://github.com/niggler/bessel and found that the JS version is faster than the straightforward Java implementation.