mIRC doesn't compile anything. Scripts are stored in memory and interpreted.
You can tell they're stored in memory by deleting a loaded script. It will still run in mIRC.
If you check
versions.txt or the forums you'll often see posts such as "I improved for routine for such and such so it now runs much faster" by Khaled, I think that's the most we can expect as a lot of mIRC's code is probably from when he was less talented. Asking for an actual compiler is a bit much, because for a start compilers are huge projects anyway; developing something to compile scripts as well as incorporating it into an IRC client would be really difficult and one or the other would most likely be neglected.
