The number of posts to be reviewed does not stop growing. Where once it was possible to clear these posts to zero, today that number has passed 700, and it was lately standing even at more than 800.
However, when I try to do my part and vote on the posts in question, I am quickly blocked by the limit of 20 reviews per 12 hours (even though the voting message says "24 votes"). It also doesn't help that I am periodically presented with "test" questions.
I do not understand where these limits of 20-per-12 came from, but I believe that they are at the root of the problem of increasing number of untreated posts in the Close queue.
My suggestion is to increase the number of reviews above 20, for example to 50 as I believe is the case now on Stack Overflow.
I would also suggest a further increase in that limit for users that have achieved the privilege level of trusted user, perhaps even making it unlimited, and also to not post test questions to such users.
The other limit of 12 hours is in my opinion less useful to change, as for many users there are normally only certain hours of the day when it is possible to contribute here. But perhaps making it 6 or even 3 hours would do just as well.
It has been answered by @Shog9 that the number of reviews is limited to 20 in order to democratically divide reviews among as many reviewers as possible.
I have been tracking reviews for the last week, and I can state that this is not what's happening. There is a small number of active reviewers that are engaged in reducing the Close queue. When these dedicated users are less active, the queue grows. When most of them are active at the same time, the queue shrinks.
My conclusion is still the same - the limit of 20 reviews blocks these community-minded users from finishing the entire queue. As @Shog9 has said, some posts in the queue have been there for more than a year.