Bram Bos's Ruismaker was originally conceived as a hardware drum machine, but it's ended up finding its way onto iOS in the form of an Audio Unit Extension.
Designed to be used within a suitable AU host (GarageBand, Cubasis, Modstep etc), this is very much a drum synthesizer. In fact, there are no samples onboard whatsoever. As such, all sounds are generated in real time; we're promised a range of "legendary" analogue and FM percussion tones.
Editing takes place on a simple GUI that gives you access to the "most important" parameters for each sound, and CPU usage is said to be low. Ruismaker is said to be optimised for small screens, offering a 'minimal' mode for iPhone 5S and SE owners.