It is mostly CPU intensive, but if you add a GPU in the picture, the load gets reduced for the CPU.

Considering one is using both CPU and GPU, CPU will do all the calculations related to the vertices in a scene. More number of vertices will put more load on the CPU (a good amount of RAM is recommended to help with tge computation). GPU will work on the rendering of the scene, meaning, it'll convert the scene into a desired output file. The load on the GPU will depend on the quality of the render.