Overhead of ring api-defaults, 3µs per request?

repl.clj

;; start repl in perf mode (e.g. -server)
(require '[ring.middleware.defaults]) ; [ring/ring-defaults "0.3.1"]
(require '[criterium.core :as cc]) ; [criterium "0.4.4"]

(defn handler [request]
  {:status 200
   :body "Hello World"
   :headers {"Content-Type" "text/html"}})

(def app
  (-> handler
      (ring.middleware.defaults/wrap-defaults
        ring.middleware.defaults/api-defaults)))

(handler {:request-method :get, :uri "/ping"})
; {:status 200
;  :body "Hello World"
;  :headers {"Content-Type" "text/html"}}

(app {:request-method :get, :uri "/ping"})
; {:status 200
;  :body "Hello World"
;  :headers {"Content-Type" "text/html; charset=utf-8"}}

;; 315ns
(cc/quick-bench
  (dotimes [_ 1000]
    (handler {:request-method :get, :uri "/ping"})))

;; 3.2ms (!!!!!)
(cc/quick-bench
  (dotimes [_ 1000]
    (app {:request-method :get, :uri "/ping"})))