Hello,
Unfortunately, the program does not support muting of a specific staff and there is no work around that I know of for this. You can send a suggestion about this to NCH developers using the link below:
http://www.nch.com.au/suggestions/
Regarding the delays, it could be what you say but how many staffs do you have on your project? You can try reinstalling the program and see if that improves. Or you can send a technical support request to NCH technical support department and see if they can assist.
Good Luck!