This might be a weird topic to even ask, I hope I can word it clearly.

In my adventures into the modding world (I'm still learning new things after a couple of years) I've come across some performance issues that I'd like to understand better.

For example, I've downloaded a couple of popular mods, Enhanced Towns and Cities, and Inconsequential NPCs, both of which have a fairly large impact on my framerate (from 60fps down to ~20-30), which I totally accept. But why? Incons.NPCs adds more people to cities, and ETaC adds more buildings and clutter to villages... Is the CPU responsible for generating objects/actors, or does the GPU take on that job?

And for that matter, my GPU (GTX 680) has never had any problems with framerate when I've installed lots of texture mods, high res meshes, etc. Even when I added a Performance ENB with ambient occlusion and a few other nifty options enabled, generally I get 60fps, always at least 50, until I enter a mod-altered area. I'm sure these things are handled differently, I'm just not sure how.

So basically I'm wondering which hardware components cover which jobs, and why these mods are having such a large effect on my fps when in some cases they are mostly just adding a handful of buildings.