Quite likely, since hash tables are stored in RAM until they are freed, either by mIRC closing or the /hfree command being used.

You should check the amount of RAM you're starting with, and see how much other programs are using. If the total RAM being used is more than about 50% of your total RAM, then your system is (or should be) using a swap file, which means disk access, which, as I've said before, is extremely slow.