I was in perf engineering at the time. we would switch between a handful of string concatenation methods every browser release. it wasn't much about real performance, but just shifting trade-offs in the jit. but google PR team was very good at running in front of the changes and pointing their overly optimized way to magazines. so they would run an array concat test that was much faster while being much slower in plus sign concat, but they often left that out. anyway, everyone drank the coolaid. 100% of the v8 performance over spider monkey was not attaching debuggers and dev tools. and sadly, mozilla had to follow. nowadays we are mostly back to square one (still some niceties from dalvik missing).
true performance improvement came much later than that.