Samsung S7: stuttering during slow motion video recording
I am experiencing the same issue on my Samsung Galaxy S7 (Exynos).
The stuttering in slow-motion video is awful and there is nothing I can do about it. 

I have also tried without SD-card, no difference. 

The best way is to minimize the number of installed apps. It will not remove stuttering completely, but might reduce it a little bit. It is somehow the intrinsic structure of Android OS that apps are always running or in standby, i.e. consuming memory and some small amount of CPU power. Any, even the slightest, extra CPU load will cause massive stuttering.