PolygonCount

Triangle Count vs. Vertex Count
On the topic of vertex duplications, it should be mentioned that vertex count is more important than triangle count. I know I'm not going to change years of habit. But vertex count is really where the rubber meets the road, not triangle count.

Overuse of smoothing groups, over-splittage of UVs, too many material assignments (and too much misalignment of these three properties), all these lead to a very different and much larger vertex count. Which can stress the transform stages for the model, slowing performance.