I've heard that Jetpack is an awful hog, so I've avoided it. It's best to use as few and as light plugins as possible. Everything Jetpack offers is available in separate packages, so unless you really "need" almost all of it...phooey!

As for image optimization, this is also a usability issue for your viewers, who might not all have the fastest Internet connection. Slow loading will not endear your users. Those using phones with limited bandwidth usage are even more penalized by encountering a "heavy" site. If one visit wipes out their monthly quota, they aren't likely to come back.

One of the reasons I love IrfanView is the batch processing it can do. A friend whose site I set up and do a lot of maintenance on uploaded so many unreduced photos that he maxed out space. I downloaded, deleted, and re-uploaded everything dozens at a time, just running one batch after another with the same settings to shrink both pixel and byte size. The re-uploaded images were just as good to look at, but took a fraction of the drive space. And the bandwidth.