Alright, I'm rather tired of people who keep saying that polycount matters as much as it does. I know you're all going to think my opinion is absurd, but please hear me out.
There are several factors at work that inhibit performance in FSO:
1. The rendering engine itself.
2. Texture size and resolution.
3. And very last, polygon count.
What many people fail to understand is that your computer's GPU processes mesh data as a linked list of vertexes, not triangles. Using this, one could also say that vertex count actually matters far more than polygon count does, though because of the widely-accepted misconception that polygon count is a measure of performance, it's best to stick to it (because of the widespread use).
What actually matters about performance is the model's texture quality. Graphics cards have maximum texture resolutions for a reason (as well as a finite amount of VRAM to store these textures in--a list of numbers such as vertexes doesn't have as much of an overhead). Higher texture filesize (less compression), combined with high resolution leads to a higher impact on performance. The Hatsheput and the Cain/Lilith HTLs found in the Media VPs are good examples of these--the performance impact of having these ships in-game comes from the high resolution (and possibly filesize) of the textures. Normal maps count as textures too, as well as specularity maps (if any).
Polygon count has very little if anything to do with performance. Vertex count perhaps, but polygon count... not so much.
Aside from that, FSO's rendering engine isn't the greatest in the world; it has bottlenecks and performance problems that other engines do not.
