OK, here’s what you need to do: switch to Mobile rendering. I’m serious. All of the fancy post-process effects and intense graphical features get stripped down in the mobile feature set of the engine. If you want fast, dirty, and cheap, that’s the easiest way to do it. It’s already pre-set and optimized for efficiency.
However, if you want to use the full feature set of the engine, just not everything all at once, there’s a few things you need to do: first, you need to disable practically all of the post process settings: no lens flares, no motion blur, no eye adaptation, no DoF effects. I think if you just use a disabling value in the post process settings it will effectively remove that aspect of post process from the render thread, freeing you up to do other things. Second, you need to disable the reflection environment from your materials. Plugging a “0” in the specular for all the materials you don’t want’ specular reflections on would be a great way to get rid of it. Screen space reflections still run in the background, though: I think these can be effectively disabled under the Screen Space Reflections in the Post Process Settings, but I’m not entirely sure. I know you can access the engine settings and bring the samples of screenspace reflections so low they are practically non-existent, so try both methods, and hopefully that removes the overhead of running the reflection environment. Third, it would be a great idea to use lightmass for precomputed lighting: placing a lightmass importance volume will build lighting caches for dynamic objects as well. It looks great, it’s not very expensive, and you get your whole GI out of it. You can place a stationary skylight to provide the ambient lighting in your level, or change the Environment intensity/color settings in the world settings (that all feeds into lightmass), along with a sun. You can place as many lights as you want, as long as they’re all static, then you’re done!
If you NEED to go all-dynamic, you can use a movable skylight to basically provide ALL of your GI lighting. If you want a moving sun, that will require a dynamic light. Just keep in mind dynamic lights (besides the skylight) use the GGX specular model for specular lighting, and it’s a fairly expensive lighting model compared to Phong shading in UDK and Gourard shading in old, old, old school engines. Plus, any dynamic specular lights not only kicks on the fairly expensive GGX specularity, but it also RE-EVALUATES the entire material of every pixel the light touches! You’re basically rendering the entire scene’s materials twice in order to have a specular sun. Once you remove the post processing, disable the reflections, and go all static GI or a basic dynamic solution, you should be running smooth. The only other way to make your game cheaper than this is to not use lighting at all and only use emissive lighting for the materials on all the objects you make, but now we’re approaching very sad levels of graphics.
Keep in mind too, the N64 had trouble pumping 30 FPS at 320x240 resolutions, interlaced! The Gamecube/PS2 were both standard definition at 640x480i, so it only renders half the screen in any given frame. Old school PC games probably ran at 1024x768, with 1280x1024 being the highest supported resolution. Running something at full 1080p or worse, 1440p, or worse, 4K, would be really bad. By default, the editor and Play in Editor both run at the maximum resolution possible. You can just run your game at those lower resolutions or use the screen percentage post process to cut back on the resolution and save from having to render such ridiculously detailed graphics all the time.
I had an old GT 640 that ran a very nice church scene, complete with stained glass windows casting colors onto the environment through lightmass, at 85 frames per second. The only thing I did was make the lighting completely static. That scene had some heavy post processing as well. This was in 1080p. Chances are your framerate is going to be GPU bound, so taking out the screen space reflections, the specularity, the fancy motion blur, the lens flares, the ambient occlusion, and cutting back on the resolution will get you a much better framerate. It will be sad to let those things go, but your game will run well on basically any computer made in the past 4 years!