Ahh the timer is a great idea, didn't think it would work, but since the timer works independently it performs nicely.

Did this setup:

Code:
dialog big {
  title "Dialog"
  size -1 -1 515 364
  option pixels notheme
  list 1, 34 14 457 322, vsbar extsel
}
 [color:red]  [/color] 
alias addline {
  if !$feof { did -a big 1 $$fread(big) }
  else { fclose big | timeraddline off }
}


Then performed the following:

//fopen big big.txt | dialog -m big big | .timeraddline 0 0 addline

Added the 50000 lines, without freezing mIRC, excellent smile

Regarding whilefix.dll, I've never really used it, just 1 or 2 times, you could post on ms.org in the comments section of the dll, and the author might reply with some feedback.

Greets