Yeah @jomaxro can add a note to next beta (which is when this officially lands) informing people to expect high CPU usage for a few hours after this upgrade due to image re-optimization.
I tweaked this today a fair bit to avoid queue starvation and issues where avatar resizing was taking way too long, but you should expect a reasonable CPU jump temporarily while stuff is catching up.
We basically needed to go to every post with images and rebake, then resize all the images. Plus resize all the avatars.
We did this cause our new algorithm for resizing is far more efficient. Plus you get retina image support and “fuzzy” image while it loads on old posts.