Recent iPhone cameras (and high-end Android too, I assume) can take video at 240fps so you won't need to buy anything extra if you have one.
Also when I transferred videos from my iPhone to my computer and watched the videos in VLC they played back at normal speed so I guess the slow down is actually recorded as meta-data either inside the video or someplace else in iOS so that it is non-destructive. In turn of course that means that I can watch the whole thing on the computer at any speed I desire but when I talked about the normal speed stuff I mean when I watch it on the phone itself.