If y ou only want it to work when you do /lag you could use a group as below.

alias lag { .enable #lag | .quote PING $ticks }
#lag off
on ^*:PONG:{
  var %lag $ticks
  echo -a * Lag: $calc($ticks - $iif($2 isnum,$2,%lag)) ms ( $+ $nick $+ )
  haltdef
  .disable #lag
}
#lag end