CPU usage

Are you optimizing locally, or via the API?
If locally, then that is totally normal, optimizing uses lots of CPU, and a decent amount of memory (depending on image size). For example, a 5MB image might use 200MB of memory during optimization, because it has to be unpacked (basically into a bitmap) before recompression.
If it causes any issues, you can disable parallel optimization, so that each resized version of an image will be done in sequence, instead of all resizes for each upload firing at once.