Cute by using older mirc versions like 6.21 instead of the most current 7.27, I get 800kb/s instead of just 70-120kb/s dcc ssl speeds. (Not that I already suspected that, since most functions that i tested in general got dramatically slower moving up from mirc 6 to 7).
At first read, this sounds a little bogus to me. First off, the slowdown on mIRC functions from 6-7 is mostly in string functions; you're barely using any, as the bulk of your data processing is going through unfiltered binary variables. Secondly, the benchmarks, while significant, only show a ~2x slowdown in mIRC when using string functions. You're showing over 4x of a slowdown, which leads me to believe your script is not the bottleneck here.