mIRC isn't a multi-threaded application (Apparently it is) mIRC scripts aren't multi-threaded anyway, so it would save them in sequence, one after the other.

mIRC 7 does seem to take considerably longer to save/load hash tables though. Here's a measurement I took of loading a hash file (36315 items):
v7.0 - 3500ms~
v6.35 - 1600ms~

However, v7 seems to save hash files differently. After saving the same data to a file, it gains 72630 bytes compared to v6.35.
Edit: This appears to be due to v7 appending CR as well as LF at new lines when saving, even without using the -b switch.

If I load the file saved by v7 on v6.35, it takes about 5100ms~ to load, but after saving it again on v6.35, it reduces file size and loads at 1600ms~ again. The load time is barely changed using either file on v7.0.
So what's going on here?

Edit: v6.35 also seems to be up to 20x faster at saving, 100ms compared to 2000ms (not averages)