You are. Only so many mobs will spawn during a wave, and each set of spawns during that wave has a timer attached to it. It’s the reason that getting the Exalted Hordes (50% faster event spawn from kills) is considered a “noob trap” because if you can naturally reach the internal cooldown for the mob spawns during a wave, having that “perk” only means you’re going to see empty screens that much more. Additionally, to add insult to injury, if you keep getting nothing but soulspires as your events, you can’t actually eliminate any of those spires efficiently or at all if you’ve got one that’s spawned during the mob spawn downtime.
If you’re able to kill mobs fast enough to consistently hit the ICD for spawns, then you are probably ready for the next compass tier. If you’re hitting that ICD at tier 8, grats, you’ve officially beaten that activity this season. 