Strange slowness for keyword-lookup in records

user> (use 'criterium.core)
nil
user> (defrecord Foo [a b])
user.Foo
user> (def r (Foo. 1 2))
#'user/r
user> (:a r)
1
user> (:a r 3)
1
user> (bench (:a r))
Evaluation count : 5918775180 in 60 samples of 98646253 calls.
             Execution time mean : 8.016731 ns
             ...
user> (bench (:a r 3))
             Execution time mean : 32.933088 ns                 ;; <= 4X higher than with the not-found arg!
             ...
user> (.valAt ^clojure.lang.ILookup r :a)
1
user> (.valAt ^clojure.lang.ILookup r :a 3)
1
user> (bench (.valAt ^clojure.lang.ILookup r :a))
             Execution time mean : 7.979451 ns
             ...
user> (bench (.valAt ^clojure.lang.ILookup r :a 3))
             Execution time mean : 7.368049 ns                   ;; <= No performance difference here
             ...


;; And yet this is the implementation of Keyword.invoke:
final public Object invoke(Object obj) {
    if(obj instanceof ILookup)
        return ((ILookup)obj).valAt(this);
    return RT.get(obj, this);
}

final public Object invoke(Object obj, Object notFound) {
    if(obj instanceof ILookup)
        return ((ILookup)obj).valAt(this,notFound);
    return RT.get(obj, this, notFound);
}