We get lots of flags. At the root of things, that's the issue.
As Lucifer noted, some types of flags may not show up right away - or show up as highly ranked - to mods due to actions taken by 10k+ users who review them, but some flags do show up somewhere in our lists right away, such as Spam/Offensive or Other. Our list of flags is sorted magically by different weighted criteria.
Some flags are acted on very quickly, such as spam/offensive flags. Those can actually be handled before a moderator sees them, by enough users all flagging. Either way, these go quickly.
Some other flag types are easy to handle, or are ones that certain moderators might really have an affinity for. Those will tend to go quickly.
Finally, when we have a backlog of flags (almost always), we might tend to let some types of flags wait for community action, in lieu of spending moderator time on them.
For instance, flags to close questions might be the last ones some of us look at, since often, regular users may eventually handle them. Moderators no longer see the "Close" flags in our queue; they will go to the Close review queue; so they remain open for a long time.
Ultimately, we don't necessarily go through the flags in an absolute, set order; the above are just some tendencies. Each moderator will have their own personal path they take. For example, I like digging through the 'other' flags, because they can be quite 'interesting', but they aren't always something that can be handled quickly like, say, 'not an answer' or comment flags.
And for anyone else reading this; you should not use an incorrect flag type just because of something you think I am suggesting here; we decline flags that are incorrect. Use the appropriate flag type, and we will do our best (moderators and 10k users) to handle them!