This feature is also -extremely- slow if there are many logs in the directory.
I haven't spied on what mIRC is doing exactly, but I suspect it's enumerating all the files in the directory and parsing the dates for the latest one. Right now I have 2200 logs in my main log folder, so mIRC freezes for about 5 seconds after I join a channel or open a query. I would suggest that mIRC try making blind attempts at finding the most recent log, by checking for today's log and then yesterday's log before resorting to a full directory search.
I think mIRC should also display the date or week or month information (if logging by date/week/month) of the log being displayed. This can either be gathered from the filename or from the Session information present at the first or last line of the file.
- Raccoon