mIRC is not the fastest language. More often than never, the easiest way to implement something is also not the fastest way to do it.

There are various tips you can use to get your script to run faster.

$iif

$iif is much slower than using a normal condition, if you are using $iif once inside an on text event, it won't make a big difference, but inside a loop with a lot of iterations, it will, use /if and /else.