You can obtain a big speed improvement by first hiding the window before scrolling through it. This way all the displaying code in mIRC is skipped. Drawback is that you can see it disapear shortly and that might frighten people smile

I'd go for qwerty's code, no long wait, no disappearing windows smile

Code:
alias selectall {
  window -h $$1
  set -u %win $$1 
  filter -wkn $1 _selectall
  window -r $1
}
alias _selectall sline -a %win $1