Searching for URIs vs. resources

[Bob DuCharme]

Imagine that I have ten million triples, and these are two of them:
  <http://w> rdfs:label "my literal" .
  <http://x> <http://y> <http://z> .

I got the impression somewhere that this query

   SELECT ?s WHERE { ?s <http://y> <http://z> }

would run faster than this one:

   SELECT ?s WHERE { ?s rdfs:label "my literal" }

Is this true, and if so is it because URIs will always be indexed and 
literals won't necessarily be? Or is it all dependent on the 
implementation?

thanks,

Bob