I am also able to reproduce this issue, however it depends on the font and script that I am using. Unfortunately I don't know of a way to improve the speed in the current implementation. Font linking is an intensive process and requires mIRC to scan all existing fonts on your system for any matching scripts or characters for every line and character that is displayed. Since mIRC is a non-unicode application, it has to perform this process manually. In a future unicode version of mIRC, this will not be necessary and should result in a faster display.