Yes i saw that before you removed it above smile, i knew it would be faster again, but i just thought id stick to /filter alone when i did mine.
Yours took 1/2 the time of mine about 1 second, just as i suspected.
I made a slight fix on the error detet line
if !$isfile($1) || ($nofile($2) && (!$isdir($nofile($2)))) || !$mkfn($nopath($2)) { return Error }

!$isdir($nofile($2)) is true if no dir is present in $2

* Now For any one who cares *
loadbuf/savebuf kicks arse
filter follows up close behind
fopen etc comes a distant 3rd
$read/write well im still waiting for it to finish!

heres my Fopen one, i did try and tweak it up more than this, but just couldnt gain any significatnt speed out of it, and the code starts looking real bizzare, with prereads before the while loop

Code:
alias Splitup2 {
  if (!$isfile($1)) { return -1 }
  if (($nofile($2)) && (!$isdir($v1))) { return -2 }
  if (!$mkfn($nopath($2))) { return -3 }
  ;
  var %i = 1, %l = 5000
  .fclose splitup.source.file*      | .fopen    splitup.source.file      $+(",$1,")
  .fclose splitup.destination.file* | .fopen -o splitup.destination.file $+(",$nofile($2),$mkfn($nopath($2)),%i,.txt")
  .timer.splitup.close.files 0 1 fclose splitup.source.file* $(|,) fclose splitup.destination.file*
  while (!$fopen(splitup.source.file).eof) {
    .fwrite -n splitup.destination.file $fread(splitup.source.file)
    dec %l
    if ((!%l) && (!$fopen(splitup.source.file).eof)) {
      inc %i
      .fclose splitup.destination.file* | .fopen -o splitup.destination.file $+(",$nofile($2),$mkfn($nopath($2)),%i,.txt")
      var %l = 5000
    }
  }
  .timer.splitup.close.files off
  .fclose splitup.source.file*
  .fclose splitup.destination.file*
  return %i
}


It takes some time still 44 seconds on my test, so dont be thinking it hung if u try it.