This is also present on the iPad Air 2, and i wonder if it occurs on the other 2GB devices (mini 4, 6S/6S+). It's super annoying and almost spoils the device for me because it occurs quite frequently on pages i visit regularly.
It most certainly is not attributable to ad blockers, because I've switched them off and I have also recreated the issue on mulitiple Air 2/iPad Pros in the the Apple store.
To recreate the issue:
Go to this rather simple site:
https://www.ozbargain.com.au/
It should load fine in a new tab; however, if you go to an image heavy page (e.g.
http://www.neogaf.com/forum/showthread.php?t=1046854&page=5), load it up, and then next in that same tab go to
https://www.ozbargain.com.au/ again, you'll see there is terrible rendering lag when scrolling down the page. If you open a new tab and load ozbargain, it will render fine, but it will continue to render poorly within the original tab.
You may not think it's a big deal, since you can get around it by simply opening a new tab and closing the problematic one, but I find it takes away a lot of fluidity from the browsing experience.