Giving a try to the latest BaseElement 16 release, thanks to its awesome performance issue notification, I discovered that I've 3 files with extra data sources that are themselves. That means than in FileA, I've an external data source which is the very same FileA.
This leads to a bunch of funky performances issues.
BaseElements is able to tell me which items are affected by this, and, in my case I've 202 of them to fix. Mainly script calls.
So the bogus extra data source creation can lead to a lot f problems. I wish that Filemaker would prevent that, or at least would pop-up a huge warning about it when it's creation can occur (I point out that I never create external data source with teh data source panel, so those bogus ones should have been created by filemaker at some point : script selection, TO creation, I dunno).
It seems an easy fix / warning for FMI to implement that can avoid lots of problem.
P.S : Try to analyze your solution with latest BE, you may have some of those that you're not aware of at all !