What speed and processor do you have in your "primary" computer?

If it is faster than a 1.2, then that might be part of the reason for the difference. However, since you state the variance is about ~30 sec., to my knowledge, your "primary" computer would have to be running at (or above) 12.0

A speed that, for a home system, I have never seen (or heard of) in nearly 30 years of working with computers.

While I don't know the details as to where/how mIRC gets it's information, my own tests show less than a one second variance between various machines ranging from a 2.0 (at the high end) to a 266 (at the low end)