How much lower did you cap it?
There aren't really any special tricks here. Rendering lag happens when OBS can't use the GPU because it's at 100%. If the framerate isn't capped, or isn't capped low enough, then when the game pushes the GPU 100% in order to try and hit the framerate it is trying for, then OBS isn't able to render its own frames, and you get stutter.
If having half of one percent of frames lag in a session is a problem (or about 3% as in your original log), then the way to do it is to reduce GPU utilization to the point that the scenes in your game that are the absolutely worst-- the most demanding-- can be rendered on your GPU at your target framerate set while still keeping GPU utilization at about 90% or so, so that OBS has overhead to work.
That can be done by: reducing capture frame size (canvas size), reducing capture frame rate, turning on vsync, capping framerate, reducing other fidelity options that reduce GPU load.
The only way to absolutely separate the GPU load of your game from the GPU load of OBS is a two PC setup, either with a capture card or use of other methods, like NDI (although that still has some system load).
The only other simple change I can think of is to make sure your GPU is in the right slot and is properly seated by checking PCI interface speed with GPU-Z, but I think if that was an issue you would not be able to run Fortnite at 144fps, so I'm pretty sure that's not it.
