I have a model that I got from SU and it is slow. I mean, you pick and wait. I cleaned it up as much as I could. What would cause this poor viewport performance? The model isn’t very large and normally shouldn’t be a problem to navigate. I already merged into a new scene, which got rid a lot of empty crap, but it didn’t help performance.
Delete something one by one until you find a culprit 
Yep, that is what I had to do. It was a pain, but I found it.
What was it? Was the object just corrupted or something?
Who knows. I deleted layer by layer until the issue stopped and then I merged it back from an archive.
sometimes its some pesky thing thing that has a million parts, max hates that