I have been able to reproduce the same delay here, however I haven't been able to find the cause yet.
mIRC registers itself as the owner of the clipboard whenever it reads or writes to it and then releases the clipboard so that it is available for modification by any other application, as per the Windows API documentation.
It may be that when clip.exe updates the clipboard, the clipboard tries to contact the previous owner, mIRC, using a Windows message, as a matter of protocol. Since mIRC is in the middle of a script loop it can't process incoming Windows messages, which results in the delay.
I can't see a way around this issue currently. That said, outputting the results to a text file instead of modifying the clipboard would probably be a better idea.
