why isn't `julia-subrect-opt` showing signs of a performance increase, when the its constituent function (for-each-pixel) does?

julia.clj

(defn for-each-pixel [constant max-itrs radius r-min x-step y-step px]
  (let [[j i] px
        [x y] (normalize-coords (double j) (double i) r-min x-step y-step)
        z (->Complex x y)
        zitrs (sq-poly-iteration z constant max-itrs radius)
        idx (dec (count zitrs))]
    (when (> idx @impure/max-idx)
      (reset! impure/max-idx idx))
    [j i idx]))
;(bench (julia/for-each-pixel (->Complex rc ic) max-itrs radius r-min x-step y-step [xt yt])))
;Evaluation count : 3825300 in 60 samples of 63755 calls.
;Execution time mean : 16.018466 µs

(defn for-each-pixel-opt [[^double r0 ^double i0] [max-itrs radius r-min] [^double x-step ^double y-step] [^double j ^double i]]
  (let [[x y] (normalize-coords j i r-min x-step y-step)
        zitrs (sq-poly-iteration-opt x y [r0 i0 max-itrs radius])
        idx (count zitrs)]
    (when (> idx @impure/max-idx)
      (reset! impure/max-idx idx))
    [j i idx]))
;(bench (julia/for-each-pixel-opt [rc ic] [max-itrs radius r-min] [x-step y-step] [xt yt])))
;Evaluation count : 59542860 in 60 samples of 992381 calls.
;Execution time mean : 1.038955 µs

(defn julia-subrect [^Long start-x ^Long start-y ^Long end-x ^Long end-y ^Long total-width ^Long total-height ^Complex constant ^Long max-itrs]
  (let [grid (for [y (range start-y end-y)]
               (vec (for [x (range start-x end-x)]
                      [x y])))
        radius (calculate-r constant)
        r-min (- radius)
        r-max radius
        x-step (/ (Math/abs (- r-max r-min)) total-width)
        y-step (/ (Math/abs (- r-max r-min)) total-height)
        calculate-pixel (partial for-each-pixel constant max-itrs radius r-min x-step y-step)
        for-each-row (fn [r] (map calculate-pixel r))]
    (map for-each-row grid)))
;(bench (doall (julia/julia-subrect start-x start-y end-x end-y total-width total-height c max-itrs))))
;Evaluation count : 22080 in 60 samples of 368 calls.
;Execution time mean : 2.746852 ms

(defn julia-subrect-opt [[^long start-x ^long start-y ^long end-x ^long end-y] [^double rc ^double ic] total-width total-height max-itrs ]
  (let [grid (for [y (range start-y end-y)]
               (vec (for [x (range start-x end-x)]
                      [x y])))
        radius (calculate-r-opt rc ic)
        r-min (- radius)
        r-max radius
        x-step (/ (Math/abs (- r-max r-min)) total-width)
        y-step (/ (Math/abs (- r-max r-min)) total-height)
        calculate-pixel (fn [px] (for-each-pixel-opt [rc ic] [max-itrs radius r-min] [x-step y-step] px))
        for-each-row (fn [r] (map calculate-pixel r))]
    (map for-each-row grid)))
;(bench (doall (julia/julia-subrect-opt [start-x start-y end-x end-y] [rc ic] total-width total-height max-itrs))))
;Evaluation count : 21720 in 60 samples of 362 calls.
;Execution time mean : 2.831553 ms