Besides the FAQ entry pinkunicorn referred to I can add a few other facts.
- The queue is not outdated due to outdated hardware or to little of it.
- The code is outdated in many ways, but it's not related to this. :)
- It's related to the API in the matter that it's rate limited. This isn't an issue under normal circumstances.
- We are not doing much about it since this is not normal, it will soon be over.
- It's not likely that you actually are affected by this, at least not related to finds you have made. Data that are being updated are done so in an order of magic priority, though it's unpredictable as well. Data on your hides are more likely to have a higher latency, if those finding them don't use Project-GC regularly.
- API issues does have something to do with it. Somewhat. I'll get to that.
We discovered a bug (or unexpected behaviour) in the API Project-GC uses to retrieve data a few months ago. It was probably more of a misunderstanding than a bug, anyway, adjustments to the API was made.
This forced us to fetch ALL data about geocaches and their logs from Geocaching.com. Normally we fetch incremental changes instead (added, deleted, updated logs). This caused a huge spike in our queue back then.
It also meant that most data was updated in a shorter time span than usual. Geocaches that hasn't been refreshed for a long time will get updated again after a certain time. This happen to be a lot of geocaches right now since most were update "at the same time" a while back. The bump this time is much smaller than last time and with time it will be even more spread out again. I would expect a new bump in a few months, though I would guess the alert will only exist for a week or two.
For the current warning, I expect it to be gone sometime this week. My guess is within 50 hours.
Bottom line is that there is no real issue here. Almost all data is updated as you would expect. You are raising a question about a warning, not an issue in itself. At least you are not mentioning that actually miss any data.
I hope my tone doesn't sound harsh, it's not meant to be. English isn't my primarily language and I have a fairly hard time explaining this in my native language, even harder in English then.
UPDATE: A screenshot that shows the queue over time, and the effect of the bumps, http://storage1.static.itmages.com/i/17/0516/h_1494960851_3831008_80a30c870c.png