EWWW and WP4.5

That’s a great question, so here goes…

The optimization in WP 4.5 does not fully optimize the resizes much more than it did before. It’s still using GD or Imagick to process the images. For whatever reason, neither of those is built to fully maximize compression on your images. Only EWWW IO can do that for you.

Basically, they did two things:
1. Drop the default quality to 82 (instead of 90).
2. Remove some metadata from the images.

Both of these changes ONLY affect resizes. The originals are left intact, as you uploaded them. The result is smaller resizes, but those resizes can still be optimized further by EWWW, just as they could be before WP 4.5.