MikeOxmall Can confirm it's definitely a CDN cache issue. At least the issue I'm experiencing.
Using Chrome developer tools I can see the response header contains "x-cache: pass" on files that download slowly, this is true even if I start the same download again after successfully downloading the full file once. I would expect the CDN to have cached the file at this point, but it isn't.
Either the server response is telling the CDN not to cache these files or the CDN is refusing to cache them for some other reason.
Files that download fast contain "x-cache: hit" in the response headers confirming the CDN has them in it's cache.