Ah, you're using the Media Foundation AMD encoder. Is there a particular reason you're using that?
Does it also do that if you're using x264 instead? Your CPU should be strong enough to do x264 and play Minecraft at the same time. Try these settings:
Output
Output Mode: Simple
Recording Quality: Indistinguishable Quality, Large File Size
Recording Format: flv (you can remux this to mp4 later if you need to)
Encoder: Software (x264)
If you really want to use an AMD hardware encoder, try using the newer
AMD AMF plugin instead. You can install it now, or wait till OBS Studio 0.16.0 comes out, since the new plugin will be including with that.