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
   61 
  179c179
   61 
  6837,6838c6837,6838
   42 
  > 118 
  6866,6867c6866,6867
   118 
  > 42 
  7028c7028
   23 
  7076c7076
   23 
  7138c7138
   113 
  7166c7166
   113 
  7226c7226
   117 
  7278c7278
   117 
  7416,7417c7416,7417
   84 
  > 53 
  7434c7434
   86 
  7470c7470
   86 
  7487,7488c7487,7488
   53 
  > 84 
  7531c7531
   18 
  7539c7539
   19 
  7565c7565
   19 
  7573c7573
   18 
  7831c7831
   116 
  7873c7873
   116 
  7937c7937
   18 
  7967c7967
   18 
  8006c8006
   108 
  8008c8008
   127 
  8018c8018
   23 
  8086c8086
   23 
  8096c8096
   127 
  8098c8098
   108 
  8152c8152
   163 
  8707c8707
   111 
  8797c8797
   111