In this case, newer versions of mIRC are working better in that ever since INI file support was changed, the number of reports of corrupt INI files decreased dramatically.

However, better does not necessarily mean faster and can often mean slower because of the amount of extra work that new features need to do to improve behaviour, security, and robustness. That is why it is difficult to compare older versions of a technology with newer versions - the number of differences are far greater than one issue.

That said, in the case of INI files, newer versions of mIRC are actually much faster once the INI file is loaded. For example, try the following script that writes and reads 10000 items. If anti-virus software is disabled, mIRC v7.54 takes 2 seconds, while mIRC v6.x takes 50 seconds. If anti-virus software is enabled, mIRC v7.54 takes 2 seconds again, while mIRC v6.x takes 109 seconds.

Code:
test {
  var %ticks $ticks

  var %fn = test.ini
  remove %fn
  var %n = 0
  while (%n < 10000) {
    var %section = n $+ %n
    writeini -n %fn %section r value
    if ($calc(%n % 1000) == 0) { echo n: %n }
    inc %n
  }

  var %n = 0
  while (%n < 10000) {
    var %section = n $+ %n
    var %fword = $readini(%fn, %section, R)
    if ($calc(%n % 1000) == 0) { echo n: %n }
    inc %n
  }

  echo tc: $calc($ticks - %ticks)
}

So, while /writeini and $readini() are initially slower in newer versions of mIRC when loading large INI files, if a script continues to use them, eg. as in the on TEXT events you are using, they are far faster. In the end, which is better really depends on your use case.