Why is the program too slow?

Hi,everyone.
I use the following program calculates Fisher's alpha from counts of 
individuals and species. The program is wrote by Prof. Kyle Harm.
However, when I run the program, it can work very quickly sometimes, 
but it can not work very well sometimes. It depends on the counts of 
individuals and species.
For example,

calc.alpha(1000,70)

[1] 17.14375

calc.alpha(10000,70)

[1] 10.15460

calc.alpha(100,7)

[1] 1.714375

But,

calc.alpha(1580,30)
calc.alpha(1000,7)

It is very slow.

So, what is the problem?
Thanks very much.
                                                     Jian Zhang


# The following function calculates Fisher's alpha from counts of 
individuals and species.
# Note that this program assumes that the true value of alpha lies 
within the range 0.001?C10000
# (a likely assumption for local assemblages of organisms).
# The function returns "-1" if there is a problem.

calc.alpha=function(n.orig, s.orig)
{
 a=numeric()

 len.n=length(n.orig)
 len.s=length(s.orig)

 if(len.n != len.s)
   { return(-1)  }

 for(i in 1:len.n)
  {
    if(n.orig[i]<=0 | s.orig[i]<=0 | n.orig[i]<=s.orig[i])
{ a[i]=(-1) }

    else
     {
      low.a=0.001
      high.a=10000
      low.s = low.a*log(1+(n.orig[i]/low.a))
      high.s = high.a*log(1+(n.orig[i]/high.a))

      if((s.orig[i]<=low.s) | (s.orig[i]>=high.s))
       { a[i]=(-1) }

      else
       {
        use.s=s.orig[i]+1
        while(s.orig[i] != use.s)
         {
          use.a=(low.a+high.a)/2
          use.s=use.a*log(1+(n.orig[i]/use.a))

          if(s.orig[i]<use.s)
 { high.a=use.a }

          if(s.orig[i]>use.s)
 { low.a=use.a }
         }
        a[i]=use.a
       }
     }
  }

  return(a)
}

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