One of the annual tournaments that we play in has 120 participants and a shotgun is the desired start. The 2018 version with 30 groups of 4 had round times of 5:30-6:00 hours. The 2019 version had 20 groups of 6 and the round times were 4:00-4:30.
The moral of the story is if you have space to move into you can move around a golf course, if there is no space to move, gridlock is inevitable. The easiest way to create space is to have tee time intervals further apart and the paradox that golfers struggle to understand is, you can start later but finish at the same time or even earlier.
The way to determine what the tee time space should be is, if the first is a medium to long par 4, how long it takes a group to be either putting out or moving to the next tee. It will vary on the number of players and the format of play. The mindset of getting as many players off the first as quickly as possible is really difficult to change.