The rule: Be super selective when choosing who to CC on emails.
When there are multiple recipients, don't default to 'Reply All'. Maybe you only need to CC a couple of people on the original thread. Or, perhaps none.
Some people like to be in the know about almost everything, but most would prefer to reduce their inbox load and only be included on topics relevant to them. When choosing your list of CCs, try to select the minimum number of people who, by your judgment, must be included in the thread.
Key takeaway: CCs are like mating bunnies. For every recipient you add, you are dramatically multiplying total response time. Tread carefully!