On iPhone the kana input method for Japanese is a 10-key keyboard with a layout like this:
あかさ たなは まやら わ
It's really a pleasure to use because any kana character is only a swipe away.
For example, if you tap and hold "あ" you can then slide left, right, up, or, down to get any vowel sound in the あ column.
う いあえ お
On iPad there is no 10-key keyboard offered. Instead there's this 55-key beast to hunt-and-peck through:
「わらやまはなたさかあ ？をり みひにちしきい ！んるゆむふぬつすくう 、ーれ めへねてせけえ 。小ろよもほのとそこお
So on iPad I use the romaji keyboard instead (English keyboard layout that converts to kana phonetically). This is about as fast as typing English, but it's still not as fast as iPhone's 10-key.
Do any third-party keyboards exist in the Japan or US App Stores that offer 10-key input similar to that of iPhone?