Well, mIRC states to cache ini files. There is also that /flushini command:
Flushes the specified INI file to the hard disk. INI files are cached in memory, so you may want to do this to make sure that your INI is updated properly.
I wondered how long and how much of ini data will be "cached in memory".
Of course mIRC will not "load" or "cache" your ini files on its own, as OrionsBelt pointed out. The inidata is storad as "physical" file.
Somewhere I read there is one, but I'm not able to reproduce a measurable time offset between first read attempt of a (large custom) inifile and further read attempts with a script. Maybe someone can clarify this?
EDIT: ok, after some time and/or using a "ram defrag" tool, there is a measurable difference:
//var %start = $ticks | echo -a read: $ini($mircini,20) | echo -a $calc($ticks - %start) ticks
//var %start = $ticks | echo -a read: $readini(servers.ini,$ini(servers.ini,2),n0) | echo -a $calc($ticks - %start) ticks
first attempt: 15, 30 or even 120 ticks
second attempt: 0 ticks