I ran benchmarkParse.php on [[Barack Obama]] with a three different hhvm builds: debug, release and release with ALWAYS_ASSERT. The overhead of ALWAYS_ASSERT was smaller than the error bars, probably less than 1%. The overhead of debug mode was a factor of 7 increase in parse time.
I confirmed that the ALWAYS_ASSERT build did actually have assertions enabled, by using objdump -t to check for symbols that are conditionally compiled.
-- Tim Starling