I use mIRC 6.14 and it takes me about 1-2 seconds to open the script editor and the options (same with closing them). (XP Pro/~1Ghz).
Make sure you are not running any resource-eating applications, check for ones that might be running in the background with CTRL+ALT+DEL.
I guess you could be using some sort of script that's eating resources too - install a fresh mIRC to a clean directory and run it with no scripts, see if it opens equally as slow with that mIRC.
I would say the computer lays with your computer rather than mIRC itself - actual mIRC bugs tend to affect ALL versions of that mIRC for all people, or it can depend upon the OS. What OS are you using?
Regards,