Earlier quoted context omitted.
It would surprise me because there just isn't that much time being spent in JavaScript on this test. Do the math. If a program spends 2% of its time in V8, 10% of its time in the network stack, and 80-something% of its time reading a file from disk, then how can you even consider that you can make it 10 times more effective by optimizing away the 2%? Even if the JVM was 100 times faster than V8, then you would expect…
The statement, "The JVM and V8 are both fast enough to be almost negligible" is flawed. OS file system caching pretty much makes disk IO negligible, and the time spent in JVM and V8 are the majority of the time. The benchmark is consistent with the system behavior, showing the difference between JVM and V8 is not negligible but substantial.
As some other commenters pointed out, if you pre-load the file contents, the discrepancy goes away. Also, if you actually limit vert.x to 1 CPU, it gets erratic and loses any performance benefits.