I am guessing it is related to dereferencing. It seems Google agrees, it's also mentioned on Wikipedia at http://en.wikipedia.org/wiki/Reference_%28C%2B%2B%29#Relatio...

[snip]...consequence of this is that in many implementations, operating on a variable with automatic or static lifetime through a reference, although syntactically similar to accessing it directly, can involve hidden dereference operations that are costly.[/snip]