I suspect this has nothing directly to do with the number of users on your CC list, but with your mail settings and the mail addresses of those users. What did you configure in Bugzilla to send mails, SMTP?
This would mean that Bugzilla directly sends a mail to each user, talking to the users mail sever etc. This can cost a lot of time and simply doesn't scale. Depending on your OS you have different options, like using the mailqueue, talking to a local postfix which will forward the mails in the background itself etc.