Must be engine exclusively. I have an And 8350 and it handled this game like a boss. Vanilla with hrdlc I used to have 60 no drops...now its modded I never see my CPU get taxed more than like 40% but the fps can't stay consistent.
CPU´s would only reach 100% usage in stress tests. Otherwise programs are made up of tiny chunks of data instead of one large continous stream. This means that the CPU performs what is requested and then moves on to the next job. So never read too much into the CPU usage levels, since it makes very little sense to use it as a parameter for performance.
The more mods you add etc. the more scripts needs to be run, and the less "room" there is for lighting calculations etc. This all means that there will be a bottleneck eventually and this means a reduction in FPS. Most of the time however the bottleneck is very small and only manifest itself as a drop in FPS rather then a huge lag spike.