The difference is that DF actually gives things like this a purpose. They effect stats. They also don't waste time graphically simulating most of this. (It used to be none, but now we do have some graphical representations of some traits, like beard/hair style, skin color, etc.)
Excellent point. But also, these things absolutely will bring the strongest computer to its knees given a long enough time or a large enough embark area.
Maybe. My understanding is they have a very minor impact on performance. Line of sight I think is still number 1, by a lot, although that's been improved greatly recently. Temperature is also somewhat bad. Both of these have fairly large gameplay impacts.
The data connected to a unit doesn't really hurt performance. It could fill up RAM if it were enormous, but the way I think the data is layed out in memory makes it very efficient to utilize. If you're performing an action on a unit anyway you've got to bring their data into RAM, and it's all grouped together so it's one big chunk of data that gets pulled in together and can be operated on.