Firefox application crashed while I was entering this bug. So doing it again, hopefully no duplicate. Looking at a treeherder log such as http://firstname.lastname@example.org/try-win64/try_win8_64_test-web-platform-tests-3-bm112-tests1-windows-build1178.txt.gz Shows a few lines being displayed and then everything seems to stall. Attempting to scroll is extremely slow. Attempting to jump to the end by swiping quickly is impossible due to everything becoming so slow. Safari on the same iPad (iPad Air) doesn't slow down in any ways, scrolling is fast and you can scroll to the bottom fairly quickly.
On my iPhone 6, the entirety of the content is displayed and scrolls fine. However I did see in console a memory pressure warning 2015-09-21 10:49:44.044 Client[649:565301] Received memory warning. 2015-09-21 10:49:44.052 [Debug] [BrowserViewController.swift:109] didReceiveMemoryWarning() > BVC received memory warning Scrolling ways down and then tapping the status bar to scroll back up to the top caused some latency and responsiveness issues