My website seems to be slower, could this be because ShortPixel is still optimizing images?
ShortPixel was designed to use as little resources as possible and only be run when users with certain roles are logged in.
That means when someone is visiting your website the load speed should be virtually the same regardless whether ShortPixel is optimizing images at that very moment or not.
If you notice, while logged in as admin (for example) that the dashboard is slower than usual it could be because ShortPixel is running in background. If you want to make sure that ShortPixel is the cause of that just stop the optimization process in Media Library/Bulk ShortPixel and see if it makes any difference.