On my 950 the FB app takes about 7 seconds to load. However, sometimes it does take longer and about 1/3 of the time the first attempt to launch it fails after grinding away for 10 seconds. It also sometimes randomly dies, but not very often.
Other annoying things: Scrolling feels... sticky? It doesn't glide along as far with a hard flick like most screens. And the keyboard doesn't quite work right either. Like, if I try to highlight a previous word in an edit box it doesn't let me. For FB or Messenger. So certain kinds of editing don't work, like trying to go back to a word and select a different suggestion. Also, this one's a huge battery hog when running for me, as is Edge.
Besides all that, the app's actually pretty awesome...?