Added type hints I suspect would make math faster here. Just eyeballing, so not sure if it's all right. Normally I would check the bytecode to look for boxing as well.

breezy.clj

(ns fun-with-quil.breezy
  (:use quil.core))

(set! *unchecked-math* true)

(def ^:constant screen-w 1920)
(def ^:constant screen-h 1080)

(defn r ^double [^long x ^long y]
  (loop [k 0 i 0.0 j 0.0)]
    (if (< k 15)
      (recur (inc k) (+ (- (sq (sin i)) (sq (cos j))) (/ (- x 0.0) 1024)) (+ (* 2 (sin i) (cos j)) (/ (- y 1024.0) 512)))
      (* 90 (abs j)))
    )
  )

(defn g ^double [^long x ^long y]
  (loop [k 0 i 0.0 j 0.0]
    (if (< k 15)
      (recur (inc k) (+ (- (sq (sin i)) (sq (cos j))) (/ (- x 512.) 1024)) (+ (* 2 (sin i) (cos j)) (/ (- y 512.0) 512)))
      (* 90 (abs i)))
    )
  )

(defn b ^double [^long x ^long y]
  (loop [k 0 i 0.0 j 0.0]
    (if (< k 15)
      (recur (inc k) (+ (- (sq (sin i)) (sq (cos j))) (/ (- x 1024.) 1024)) (+ (* 2 (sin i) (cos j)) (/ (- y 0.0) 512)))
      (* 50 (+ (sq i) (sq j))))
    )
  )

(defn setup []
  (smooth)
  (no-loop)
;  (color-mode :hsb)
  )

(defn draw []
  (doseq [x (range screen-w)
          y (range screen-h)]
    (stroke (r x y) (g x y) (b x y))
    (point x y))
  (save "breezy.png"))

(sketch
  :title "breezy"
  :setup setup
  :draw draw
  :size [screen-w screen-h])