addAll is often more efficient than calling add many times individually.

Arrays.asList only creates a single lightweight object that wraps an array so I don't think that would be much of a problem.  http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/Arrays.java#Arrays.ArrayList

But that being said, I don't see an obvious performance advantage to using addAll for a plain unsynchronized HashSet http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/AbstractCollection.java#AbstractCollection.addAll%28java.util.Collection%29