HP has just issued the HP Prime app for iPhone and iPad! It works fast. There are several minor bugs, but that's to expect, since the new appel guidelines imply that the code had to be converted to 64 bit from 32, and that all characters are stored with 32 bits (which allow much more complex character sets).
The app can be found in the App store. Be aware that there was a version for Android since several months ago.
You have here a demo of the app: