OK, sorted!
Instead of all 400 pictures reloading onto the page every time you check your progress, there's now just a link to open up a separate page full of jigsaw pictures. Once you choose one it returns you straight back to the jigsaw puzzle page with the new on in place. It's sped up the page no end!
Oh.... and there's also a larger "
Check progress" button above
and below the puzzle, for further convenience.