What is the correct action? Should one wait for a question or answer to appear in the review or just go ahead and edit or flag it as soon as possible? I get the impression, from the slider, that there's something to be "gained" by dealing with questions or answers that are in the review queue.
Make Ask Ubuntu better even outside of review!
If you see a post that should be flagged, please always flag it immediately.
Similarly, if you see a post that should be upvoted, downvoted, retagged, edited, rolled back to a previous edit, close-voted, reopen-voted, deleted, or undeleted, and you have enough reputation to perform the action, you should do it, whether or not a review queue is the place where you're seeing the post.
The goal of review is to give posts the attention they need. It should never discourage you from contributing--that's the opposite of its purpose.
What the Review Sliders Mean
In review, you get to see how many previous reviews (of that type) you've completed. Others can see this too.
There are also badges conferred for reviewing, based on the number of reviews. This ranges from a bronze badge when you review your first post in a particular queue, up to a gold one when you review your thousandth.
However many posts you flag, improve, vote on, ..., outside review, there will still be plenty more to review. Please don't let your desire for big review numbers and badges get in the way of contributing to the site!
By the way, when you do those things, that's recorded too. The number of times you've voted on posts is visible by all (though not what posts or how you voted--that's private). The number of edits, and the edits themselves (here's mine), are public, as are edit suggestions (here's mine). All your flags, and how many, and if they were found helpful, disputed, or declined, are remembered (though only you and moderators can see that).
Whether or not, or to whatever degree, your contributions to improving the site are through the review utility, they will not go unrecognized, nor will they be forgotten (unless your delete your account or Stack Exchange goes bankrupt and its assets are purchased by a data-center-destroying death-fireball testing company).
Thank you for all your contributions to improving content on the site (currently including about a hundred edit suggestions, twice that many edits, and almost 600 votes), whether made through review, or otherwise!
How long before a post appears in the review queue?
Posts appear almost immediately in review but there are several reasons why you might not see some of them....sometimes ever.
I believe that at least two 2k+ reputation users needs to have it reviewed before it's out of the queue. (can't find the source at this moment)
No, why? If there's an action to take you should do the right thing. The only reason I can think of to wait for it to turn up in the review queue are the badges like the Reviewer badge, but there's chance you'll miss it there because others have reviewed it already.
If the flagging needs attention soon, like really bad behaviour of users, then do it straightaway. If it's just a low quality answer you come across, you could just wait for a later moment.
True. That's not really a good design in my opinion, as it encourages people to not take actions on Q&A they come across outside the review queue.
The "goodies" you earn are shown once you click on the slider:
In my example, I comment a lot on questions before they appear in my review queue. I really don't mind that much about the badges, as they will come some time later.