If your rendering gpu, or cpu is maxed out you're choking the encoder. You need to lower the encoder settings in 264x if your cpu is maxing out, and if the gpu is maxing out you need to lower the game detail, possibly the resolution too. I suggest software like MSI Afterburner to see your gpu usage, and the bundled software riva tuner can also possibly limit the fps of the game to say 30 fps so you can capture it as many people stream at 30 fps anyway. You can't expect a game to capture well when the gpu is maxed out, you are created a bottleneck. Only in cases of people having a dedicated streaming computer with a capture card can you not worry about that.
Also an update to Win 10 has made obs use a ton more usage, over double, I made a bug post about it, but nobody has responded. Obs by itself was using 23% of my gpu doing nothing now, and that also affects my games performance. So take a look at how much actual resource usage obs is using, not just what it reports on it's stats.