From the User Guide. Normal is 30 FPS, slo-mo is 120 FPS so I'd guess it's quarter speed.
“Shoot video
With Camera you can shoot video, including slow-motion and time-lapse videos.
Record a video. Choose Video, then tap the Record button, or press either volume button, to start and stop recording. You can choose to capture 720p or 1080p video at 30 fps (frames persecond). On iPad Pro (9.7-inch) you can also capture 1080p video at 60 fps and 4K video at 30 fps. To change video recording settings, go to Settings > Photos & Camera > Record Video.
Take it slow. (iPad Pro, iPad Air 2, iPad mini 4) Choose Slo-mo to capture slow-motion video at 120 fps. Later, when you edit the video, you can set just a portion of it to play back in slow motion, and the rest plays back at regular speed. On iPad Pro (9.7-inch), you can choose between 120 fps and 240 fps. To change Slo-mo settings, go to Settings > Photos & Camera > Record Slo-mo.”
Excerpt From: Apple Inc. “iPad User Guide for iOS 10.” Apple Inc., 2016. iBooks.
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