mirc stores settings in %APPDATA%\mIRC (click start -> run and type that in and hit enter to see the directory). You'll want to clear out the directory completely. You'll also want to completely clear out the installation directory (defaults to C:\Program Files\mIRC) in case you have it setup to load extra files from there.
The registry settings should not be relevant to load times.
What version are you attempting to load? What operating system?