I'm not sure, but I think I use 3 minutes or so for 2000 mp3s.
Anyway, I got this "aha"-feeling. You write a new line to a textfile (/write) each time it finds a new mp3. For each time, mIRC opens, writes and closes the file.
The solution is to find the files ($findfile) and add files (/aline?)into a @window ehich is hidden and sorted (/window -hs @window), then take a /savebuf and save it to a txtfile. That is what I'm using
[It seems Cold and I agrees. I just used 8 minutes to say the same thing]
