I have been playing again a lot recently and I feel like there needs to be a greater range of options for mission speed.
To begin with, I would love to see a 2 minute option for spawning speed, as I find 1 Minute too fast and 3 minutes too slow.
Secondly and more importantly , I would love a random spawn time. Where you could select a max time interval and minimum interval. For example, it could be minimum of 1 minute and a max of 5 minutes. Everytime a mission spawned, it would reset the timer, and a new mission could spawn between 1-5 minutes. This would give the game a little more realism in the form of randomness, but would also increase the spawn rate of missions, without swamping you with missions.