Why are some of my old decision tasks stuck waiting to get picked up while newer decision tasks immediately get picked in cadence?

Question

When my cadence server has too much load, I can see continuous decision task timeouts and sticky task lists dont help, as they also timeout. But I observed, some of my old workflows were stuck on the decision task for 24 hours till the workflow eventually timedout, whereas newly spawned workflows were able to finish decision tasks with some delay in the order of seconds.

I have 50 parititions and 125 pollers for this task list. Since tasklists are basically queues, how is it possible that older decisions are continuously stuck, while newer ones pass through ?