Thanks for testing it out. Based on the size of your files and looking at your script, it looks like there are thousands of sections in your INI files and each section has one item. Is that the case? Can you post the first ten lines of your INI file?
That said, it is very likely that the INI files will be faster in older versions. The reason for this is that the INI file routines were updated a long time ago due to users reporting issues with corrupted INI files. mIRC was changed to manage INI files itself, instead of relying on Windows to cache and update INI files in the background. This means that, at the cost of being more robust, larger INI files will indeed be slower. This cannot be changed and it cannot be sped up because mIRC has to literally load and parse an entire INI file before it can manage it.