I log everything in all channels (except control codes). I also have an alias that allows me to find things in the log via mirc, though it uses Regex, so it may not be helpful to you. One of the logs is almost 3 million lines long and it take 30 seconds for a simple Regex and as long as 10 minutes for a complex Regex.

Code:
/scanlog {
  var %logfile = C:\Progra~1\mIRC\logs\ $+ $active $+ . $+ $network $+ .log
  var %get.chan = $active
  window -e @CheckChan
  echo @CheckChan 10Checking %get.chan for the phrase3 $$1-
  filter -fwpg %logfile @CheckChan / $+ $$1- $+1 /i
  if $filtered = 0 { echo @CheckChan No matches found! | join %get.chan | halt }
  echo @CheckChan $filtered entries found!
  join %get.chan
}


You can alter that as you see fit. I could add a few more things to it, but it works for my needs. Enjoy.

Yar