Just remember you are not typing in the Romaji, you are just activating the IME… so, eye on screen, finger on keyboard, be a touch typist.
Japanese IME is configured so that most Kana can be made from two key presses. So, not only NN, I usually type SI, TI, TU as well.
I recently found a keyboard shortcut for typing certain loanwords, using Q; but it is indeed rare.
クォーター