Thanks Michael for the advice and telling me the right direction.
Let me help this thread with some background information.
The DJ Spooky app is a rebranded version of DJ Mixer Pro by MusicsoftArts. They always had some smoothness and audio quality problems, so you need to use the latest iOS devices to have an okay performance.
Traktor DJ is from Native Instruments, the biggest audio dev house and leader of the DJ software market. It's smooth and has better quality, but designed for the "sync heads" as many says.
The App Store has many DJ apps and almost all is capable to do an occasional event with (like a house party). DJ Spooky (as a pro DJ person) has high skills and he'd be able to play any event with any of those apps.
But the point is, those apps are not designed for "continuous" usage, to make a "DJ living" for example. They don't support MIDI controllers, but you can use an external USB audio interface to have proper separate stereo output channels for master and pre-listening.
Cheers,
Gábor