Priority Rules Contribute to the Backlog
One factor contributing to the huge backlog in the close queue may be the priority rules used for presenting the questions for review. It appears that recently-flagged questions dominate what is presented. Questions are entering the queue at about the same rate as they are being cleared. However, the queue has built up over a long period.
Reviews Don't Reach Completion
One side-effect of this process is that the review starts for a question, it receives some number of votes, but is then buried in the queue by newer questions and the review is never finished. In a sense, the voting has been wasted because despite the reviews, the question becomes just another item in a stagnant collection that is in varying states of completion. The queue contains questions that are a few votes short of disposition (close or don't close), but they never make it back to being presented.
Reviews Are Not Used Efficiently
There is another aspect that affects how quickly the queue is cleared. There are "X" reviews performed per day, and that is what is available for clearing the queue. How those reviews are used can make a big difference in how far they go.
On some questions, the close issues are clear-cut and opinions unanimous. On others, opinions are mixed. On clear-cut questions, five reviews produces a disposition. On questions with mixed opinions, it can take close to ten reviews. The available reviews go twice as far in clearing the queue when they are focused on clear-cut questions. Beyond just efficiency, the disposition of clear-cut questions is more important than for ones lacking agreement.
These two factors suggest two things that can be done to more quickly clear the queue.
When selecting questions to present for review, mix in a certain percentage of older questions that are just a couple of votes short of a disposition.
The second strategy applies to both new and old questions. Questions would get prioritized based on the expected number of additional votes needed to reach a disposition. This is based on the question's ratio of close votes to keep-open votes to date. Clear-cut questions that can be resolved with the fewest expected additional votes get the highest priority.