I think he means INI file extensions, mirc offers INI and MRC by default. When you save it has a .ini, mirc likes to format the file like this:

n0=line 1
n1=line 2
n3=line 3

However, if you save as anything else, it will be saved as plain text:

line 1
line 2
line 3

So in theory, .mrc should be faster to load than .ini, but mirc keeps scripts in memory, so events like on join should be just as fast either way.