V8 gets C++-like performance when it is getting code that JITs very well. This is typically highly-numeric code, even sometimes tuned specifically for the JIT. This tends to cause a great deal of confusion when people see the benchmarks for that highly numeric code and then don't understand why their more conventional code doesn't get those speeds. It's because those speeds only apply to code you're probably not writing.

If you are writing that sort of code, then it does apply; the speed for that code is real. It's just that the performance is much more specific than people think it is. In general V8 tends to come in around the 10x-slower-than-C for general code, which means that in general it's a very fast scripting language, but in the landscape of programming languages as a whole that's middling single-thread performance and a generally bad multiprocessing story.