Using Safari?
I am using Safari, I turned off WiFi and loaded a page with 100 thumbnails, iPhone 6 iOS 8.x is indeed very slow even when on WiFi and LTE, not seen in the previous iPhone models, Agreed. When the page loaded, it was slow, did not load pieces and parts of the site, loaded at once and images are missing or, replaced with black or ½ way black, as you stated.
Refreshing the page loads everything immediately though not the images.
Since I created the site I am visiting, I know I'm using custom CSS to wrap the thumbnails to display as a "Polaroid" look, rounded corners. While typing here to suggest the loading delay on LTE is causing images to be skipped, I noticed that the page automatically kept loading though the images skipped and the images replaced with black or half and half did not load even after refreshing.
I thought this would be website-design related though a refresh would always kick in the images missed.
In my opinion, the images are not website-design related, this is due to a strong and noticeable delay with iOS 8.x. I have a Microcell with AT&T, it's a 4G LTE Microcell which I connected to for this test and, indeed, the iPhone 5 loads fine, the iPhone 6 has a delay which causes this issue.
It's not just your phone, mine is doing it too and doing this with full bars from an LTE Booster from AT&T. iOS 8.x related certainly, in my opinion!
