Think of the math in dollars and cents.
8 minute interval = 30 golfers per hour starting
12 minute interval = 20 gofers per hour starting
If the course charges $40 per golfer that is potentially a $400 per hour loss of revenue. How many businesses do you know of that could take that hit and stay in business?
Even dropping to 10 minutes is still $200 per hour in lost opportunity for income.
My home course staggers 8 and 9 minute intervals from open until 10:30, then 9 minutes for the rest of the day. The difference is approximately one foursome per hour. In midsummer that means that they get 4 more players per hour for the first 5 hours - 20 more players at $40 each is $800 a day. That alone covers most or all of the salaries paid to the shop staff and seasonal workers.
Playing with the intervals is playing with money, not just time.
Why? If the course is an easy course with few situations where a ball might be lost, then there is no reason at all to think that 8 minute spacing should be a problem. I know that my typical group can play their tee shots, move up the hole, play seconds and be out of range in about 6-7 minutes without being rushed. I find that with 9 or 10 minute intervals, we usually have to wait on the first tee for 2 or 3 minutes after the group ahead is well out of range, sometimes until they are almost done putting before our starting time comes up.
Give some actual reasons for your bald statement.
Keep track of offending parties then don't allow them to have early tee times. Let the faster guys and girls go first in the morning then let the hackers. Tell everyone tee times are determined by play speed history, which you keep track of. When 5 hour fivesomes wanna play, let them tee off at 1 or 2 pm. They'll speed up when it starts getting dark.
Can't do it on a public course which caters to a customer base of thousands, many of whom may only play that course once or twice in a season. No possible way to track them. And on a course which is partially tax funded, you can't give preferential treatment to anyone. Unrestricted fivesomes are a bad idea at any time.