Something I've noticed in Iframely is that the number of API hits has been steadily increasing over time. I reached out to Iframely, and they noted that NodeBB appears to be making the majority of the API calls.
I've taken a look at your hits, and as I see the more from API calls than from iFrames. It means your NodeBB is making the most hits.
The way NodeBB folks have it implemented is that they keep the cache for 1 day in server's memory. If you restart the servers, for example, - the cache will be reset. Also, if your RAM is limited, some of the records can be evicted from cache prematurely.
We did helped NodeBB team develop the plugin - but from the point where the link is already extracted and till the point before it needs to be stored. So, basically - connecting to API and UI is our part. However, if you have a pull-request for longer cache times or eliminating the older links - my team should be able to review, merge and push it to NPM.
I am far from a developer, so I'll ask if you guys can take a look at the plugin and try and add some method of lowering API hits. Perhaps if a post is older than x days, don't attempt to cache the Iframely hit until someone loads that post. Maybe going a step further and removing the embed if the post is over a certain age.