[kentonv]

You are right to distrust these claims. The reality is that threads can be significantly faster than async -- async code has to do a lot of bookkeeping and that bookkeeping has overhead. OTOH, threads have their own kind of overhead that can also be bad.

The slide deck that bysin linked above is pretty good:

http://www.mailinator.com/tymaPaulMultithreaded.pdf

This is by Paul Tyma, who at the time worked on Google's Java infrastructure team with Josh Bloch and other people who know what they're doing. Apparently he found threads to be faster in a number of benchmarks.

Ultimately which is actually faster will always depend on your use case. Unfortunately this means that general benchmarks aren't all that useful; you need to benchmark your system. And you aren't going to write your whole system both ways in order to find out which is faster. So probably you should just choose the style you're more comfortable with.

Async is kind of like libertarianism: It works pretty well in some cases, pretty poorly in others, but it has a contingent of fans who think they've discovered some magic solution to all problems and if you disagree then you must just not understand and you need to be educated.

(Note: The code I've been writing lately is heavily async, FWIW.)