I hadn't looked at it from the angle of whether the disk writes were needed, but it's definitely a good idea to avoid disk writes whenever possible, as that's always much slower than writing to a @window or a hash table.
But OP should take a look at that /speedwrite alias in the above link. There's a big difference in speed of disk writes when you tell your Antivirus to avoid monitoring disk writes to the folder where mirc.ini is located. This also affects the time it takes mIRC to write other things, such as the occasional writes to disk of mirc.ini and the variables file.
