Help optimizing array iteration

Folks,

Am I doing everything I can to optimize the code below? 

I'm copying  a RGB byte array into a RGBA byte array. I need to do
this because my partially translucent JPEGs have the mask stored in a
separate file. I tried converting JPEGs to PNG so that I could store
the alpha in the image but even without the alpha layer the PNGs are
at least twice as big as the JPEGs. It's important to me that the
images take as little space as possible.

The arrays are all foreign byte arrays, i.e. #<Pointer to type
(:UNSIGNED :CHAR)> and there are at most 427,172 iterations in that
loop.

(in-package "CL-USER")

(eval-when (compile)
  (hcl:toggle-source-debugging nil))

(eval-when (compile load eval)
  (defvar *optimize*  '(optimize (safety 0) 
			(space 0) 
			(debug 0) 
			(float 0)
			(hcl:fixnum-safety 0)
			(speed 3))))

(defun cocoa-build-sprite (sprite image mask size)
  (declare #.*optimize*
	   (type fixnum size))
  (loop for i fixnum from 0 below size
     for ii fixnum from 0 by 3
     for si fixnum from 0 by 4
     for alpha fixnum = ii
     for sr fixnum = si
     for sg fixnum = (1+ sr)
     for sb fixnum = (1+ sg)
     for sa fixnum = (1+ sb)
     for ir fixnum = ii
     for ig fixnum = (1+ ir)
     for ib fixnum = (1+ ig) do
     ;; copy color channels
       (setf (opengl:gl-vector-aref sprite sr) 
	     (the fixnum (opengl:gl-vector-aref image ir)))
       (setf (opengl:gl-vector-aref sprite sg) 
	     (the fixnum (opengl:gl-vector-aref image ig)))
       (setf (opengl:gl-vector-aref sprite sb) 
	     (the fixnum (opengl:gl-vector-aref image ib)))
       (setf (opengl:gl-vector-aref sprite sa) 
	     (the fixnum (opengl:gl-vector-aref mask alpha)))
       ))

    Thanks, Joel

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