ssubst on long strings

Hi again,

I have implemented longssubst which
does what ssubst does and works on strings
larger than 5 kilobytes.

Unfortanately my code is much slower.

Could someone tell me why?



longssubst(nsub,osub,str) :=
   block([out,len,nlen,olen,item,i],
   len : slength(str),
   nlen : slength(nsub),
   olen : slength(osub),
   out : "",
   for i : 1 thru len-(olen-1) do
     (
     item : substring(str,i,i+olen),
     if item=osub then
        (
        out : concat(out,nsub),
        i : i + olen-1
        )
     else
        (
        out : concat(out,substring(str,i,i+1))
        )
     ),
   for i : len-(olen-1)+1 thru len do
     (
     out : concat(out,substring(str,i,i+1))
     ),
   return(out)
   );


    Fabrizio