Q about Exercise 13.3(b) in "ANSI Common Lisp"

Has anyone else worked on exercise 13.3(b) in "ANSI Common Lisp"? I approached it by declaring all the variables in Figure 9.2 as long-floats:

  ; Use declarations in math utilities:
  (defun sq (x) 
    (declare (long-float x))
    (* x x))
  
  (defun mag (x y z)
    (declare (long-float x y z))
    (sqrt (+ (sq x) (sq y) (sq z))))
  
  (defun unit-vector (x y z)
    (declare (long-float x y z))
    (let ((d (mag x y z)))
      (declare (long-float d))
      (values (/ x d) (/ y d) (/ z d))))
  
  (defstruct (point (:conc-name nil))  
    x y z)
  
  (defun distance (p1 p2)
    (mag (- (x p1) (x p2))
         (- (y p1) (y p2))
         (- (z p1) (z p2))))
  
  (defun minroot (a b c)
    (declare (long-float a b c))
    (if (zerop a)
        (/ (- c) b)
        (let ((disc (- (sq b) (* 4 a c))))
          (declare (long-float disc))
          (unless (minusp disc)
            (let ((discrt (sqrt disc)))
              (declare (long-float discrt))
              (min (/ (+ (- b) discrt) (* 2 a))
                   (/ (- (- b) discrt) (* 2 a))))))))

However, the running time slowed down as a result of this change, rather than sped up.

Note that I used long-floats, rather than double-floats, because, when I used double-floats, the output was slightly different (only off by 1 on some values) than the original version in which variables were undeclared. However, the output was slightly different with long-floats, too, so I guess that didn't help.

Anyway, should declaring these variables make the program faster, or am I doing the problem incorrectly? And why is the output slightly different if I'm using the largest size floating-points? Below is the diff:

  diff "spheres.pgm copy" spheres.pgm
  125c125
  < 62 
  ---
  > 61 
  179c179
  < 62 
  ---
  > 61 
  6837,6838c6837,6838
  < 41 
  < 117 
  ---
  > 42 
  > 118 
  6866,6867c6866,6867
  < 117 
  < 41 
  ---
  > 118 
  > 42 
  7028c7028
  < 22 
  ---
  > 23 
  7076c7076
  < 22 
  ---
  > 23 
  7138c7138
  < 114 
  ---
  > 113 
  7166c7166
  < 114 
  ---
  > 113 
  7226c7226
  < 118 
  ---
  > 117 
  7278c7278
  < 118 
  ---
  > 117 
  7416,7417c7416,7417
  < 85 
  < 54 
  ---
  > 84 
  > 53 
  7434c7434
  < 85 
  ---
  > 86 
  7470c7470
  < 85 
  ---
  > 86 
  7487,7488c7487,7488
  < 54 
  < 85 
  ---
  > 53 
  > 84 
  7531c7531
  < 19 
  ---
  > 18 
  7539c7539
  < 18 
  ---
  > 19 
  7565c7565
  < 18 
  ---
  > 19 
  7573c7573
  < 19 
  ---
  > 18 
  7831c7831
  < 117 
  ---
  > 116 
  7873c7873
  < 117 
  ---
  > 116 
  7937c7937
  < 19 
  ---
  > 18 
  7967c7967
  < 19 
  ---
  > 18 
  8006c8006
  < 107 
  ---
  > 108 
  8008c8008
  < 128 
  ---
  > 127 
  8018c8018
  < 24 
  ---
  > 23 
  8086c8086
  < 24 
  ---
  > 23 
  8096c8096
  < 128 
  ---
  > 127 
  8098c8098
  < 107 
  ---
  > 108 
  8152c8152
  < 164 
  ---
  > 163 
  8707c8707
  < 110 
  ---
  > 111 
  8797c8797
  < 110 
  ---
  > 111