I've noticed that if you use the 2x or 3x image in Mosaico, which is a good idea, the image in your mailing will be quite large. Is it possible to change the jpeg compression setting to make these images smaller (in code is fine, if that's where the setting is)?
Sending a 600px image and 2x results in a 1200px 200-250KB jpeg image if you have 16:9 photo. 3x is much worse. An image heavy newsletter could end up with a few MB of images. More jpeg compression wouldn't hurt here in general. Since these images aren't shown in their native resolution anyways, I think you could compress these even more than usual.
Is this done in Mosaico or in core?