I have noticed that WP does have the bad habit of recompressing Full Size images during uploads. I mean, it does make sense creating several smaller versions of each uploaded image, applying a default 90% JPEG quality setting, but why compressing twice also the Full Size images, as they are fine just as they are?
My problem is that usually 90% of the large JPEGs I’m uploading through WP are usually set to a 50-75% quality to decrease their size, so when WP re-compress them it causes a further loss in quality and their size is being bloated between 100-200%.
Is there any way to keep WP from changing the size and quality of Full Size JPEGs?
Using a filter for jpg_quality will hellp you to set your quality for uplaoded images.
In most good implementations of Image Optimization you can find a various usae of ImageOptimization tools such as optimng/jpegtran etc, there a lot of plugins that help you to do that.