Quick answer: I had Jetpack installed and it was serving my images from wordpress servers and not my own.
I’m no Wordpress expert, but I know a few things about hosting. This little blog was running way too slow for its size (and that it’s hosted by the ever speedy unitedhosting!) So I set about deleting plugins, themes and just about anything I didn’t need. And there was a lot to remove – the usual curse of tech over substance. Nothing seemed to impact. And in the midst of this cleanse, I realised all my pictures seemed to be hosted on a different website (if I clicked on them, it took me to a page starting i0.wp.com/. It seems when the Jetpack plugin is installed, and you used the mosaic gallery, images are served from the Worpress Content Delivery Network (CDN). Which is odd.
Jetpack was the last plugin for the chop, and after hitting the kill switch, my site suddenly starting responding much more quickly. As my sites mainly made up of images, it was the pictures hosted on the CDN that were slowing things down. Which is the exact opposite of what should be happening. So the lesson learned? Don’t use Jetpack if your host is super quick! Also, I’m not sure I like the fact that wordpress were uploading my images to their server (I’m sure it’s covered by the terms I didn’t read when I installed Jetpack).