it's a dynamic difficulty thing. The more you continously combo, the harder the game gets. If you miss a couple of moles, it becomes slower. On higher combos I have seen that they can almost occupy half or more of slots, with a spawninterval of 0.5 seconds.
I might seperate it into two modes, one in which this has 0.5s as smallest interval, and one in which it is 1 s. The game starts with 2.5s interval tho.
Thanks for the feedback.