This thread has all the informations you need:
https://forums.mirc.com/ubbthreads.php/ubb/showflat/Number/262262/Edit:
/fupdate's main idea is to control how fast lines are displayed in mIRC's windows, by reducing the amount of times the display updates.
/fupdate's parameter (0-100) is a maximum delay that will be waited for before the display is updated, but there's a maximum number of lines per second that is calculated, after which mIRC will stop waiting to update the display, so even if you specify 100, mIRC may never wait that long
