I can only suspect what's going on since you provided no code and/or a test document.
First off, some questions:
What is your exact version of InDesign on what OS and on what machine?
It could be, that inDesign is calculating the proxy images for the placed graphics if you visit a spread the first time.
And that is taking its time. Could that be the case here?
ID CC 2015.4 Release
188.8.131.52 x64 Build
I don't know if code snippet would be too helpful, but here's a very slimmed down one (this assumes the insertion point for the graphic (myRng) has already been determined and the graphic file itself (myFile) selected - most of the code is wrapped up in working out those two things).
var myRect = myRng.rectangles.add(); var myImg = myRect.place(myFile); myRect.fit(FitOptions.FRAME_TO_CONTENT); myRng.contents = "";
I had thought that it only occurred when an object style was applied, but that doesn't seem to be the case. I've also cut the document down to just 5 pages and half a dozen images - same behavior (although perhaps a bit less lag time).
Your theory is good and makes sense to me. I wish I could test it, but changing the Display Performance to Fast rather than Typical has no effect on the images. They are EPS images, and I've double-checked that they are set to grey out on Fast. But, it doesn't happen. I also can't apply it locally via an everyItem() scripting command; the only way I can get the images to grey out is by selecting them manually and giving them a Fast Display Performance - but at that point they've already been "read" so it doesn't tell me anything.
Unless anyone has a different insight, I'll probably assume you're right and that it's not fixable.