iPhone 6s 4k video dropping frames

My new iPhone 6s 64GB model with 54GB of free space is dropping frames on many 4K recordings. At first I thought this was a playback issue on my computer, but I have verified this issue is with the recordings themselves. In some cases is looks like it dropping down intermittently to 15fps with an occasional freeze. Other footage is smooth with occasional drops.

Is anyone else seeing this behavior? I am hoping it is an issue with my phone, specifically, so I can get it fixed or replaced.