Re 2)

  set wildmode=list:longest

EDIT: Re 3), there's something weird going on.

I just tried the benchmark from the article that you linked ("1000000aaeou<ESC>"), and it really did not complete in a useful timeframe. However, when I insert the text once, yank it, then paste it with repeat ("iaeou<ESC>^y4l1000000p"), the operation completes instantly, and I can still browse the file without any slowdown. Undo/redo across this operation seems to take some seconds, though.