How much CPU is used depends on many different factors. For example, if your profile is using hardware encoding with a supported hardware encoder (you logs will tell you that), the core conversion may be handled by the GPU. However if your profile is asking for tasks like say decoming or deinterlacing which you GPU man not support, then it falls back to the CPU for that. Then there are other tasks, like metadata processing, remuxing, commercial removal etc which are purely CPU intensive tasks. GPU’s for the most take off the bulk of encoding processing from the CPU but there may still be things the CPU needs to do which the GPU does not depending on the profile and hardware.
For the rest of the CPU work you can still limit how much CPU MCEBuddy uses using the tools as explained in this post so it doesn’t impact the responsiveness or resources available for other tasks in your computer: