I am almost certain that this functionality is already built in behaviour of Unreal Engine 4. There is absolutely no reason for it to not do this by default. You can even notice it at work if you have things like particle systems that emit light, as soon as you are not looking at them they turn off and stop computing, notably because you can see their influence switch off as soon as they are not within your peripheral vision.
With that in mind, we need to get to the bottom of what is causing your performance issues. Static meshes, generally speaking, are not expensive things to have in a scene, assuming that you have optimised them. If your static meshes are of a sensible poly-count, are set to static, use 1k textures predominantly (2k for larger hero assets), and are all using a single master material it is unlikely they themselves will cause problems.
Other things can really kill your frame rate, like dynamic/stationary lights, use of expensive functions in materials, blueprints with excessive functionality, overdraw (this is a huge one, especially for foliage, check your shader complexity view for this one).
Of course, I am only guessing here, since I am not sure what your performance issues are rooted to. utilising the “GPU Visualiser” will show you exactly what is eating up your processing power. Also consider having an FPS counter on your screen, and looking all over your environment. It will spike when looking at high performance hot-spots. If it is constantly spiked, you may be using excessive rendering features on a machine that can’t handle it, “Cinematic Rendering”, “Volumentric Fog”, “Ray Tracing” etc…
If this only occurs at runtime, you may have a blueprint that is drawing excessive amounts from your Ram or Processor. Such as a For-Each running through a tick. I wont come up with examples since there is a million things that can cause these issues within BP.
But look into the GPU Visualiser to see what in particular is drawing so much Power, and if you report that back, we can give a go at narrowing that down even further.
Edit: From your description, I bet overdraw is killing performance. This is often the case with Foliage. Packing it too densely can cause this issue, and also using a single Quad with foliage on it is really really bad for this reason, since it creates a lot of empty transparent space on the plane, and you’ll want to minimise this space as much as possible, by modelling somewhat to the shape of the foliage. Oddly, in this case, adding more Geo to better fit the grass, will optimise your project more. Since Overdraw is more expensive than Geometry. But this will show if you look at your scene with the “Shader Complexity” View Mode.