Transparency & Performance
Rendering lots and lots of objects with transparency on top of one another, especially if using lit transparency, can very quickly become a rendering / performance bottleneck.
The term used to describe this problem is called Overdraw and overdraw occurs when you have lots and lots of objects with Transparency rendering on top of one another.
The reason that overdraw can cause such a performance problem is because the rendering cost to render transparency becomes more and more expensive for each layer of Transparency that you introduce.
To help you better identify if this issue is happening and where, UE4 has a special view mode called Shader Complexity mode.
Shader Complexity mode shows you how complex a given surface is to render by using colors to show complexity.
The more green that you see, the cheaper things are to render and the more red that you see, the more and more expensive things become to render.
To enable this view mode, all you have to do is the following.
To enable Shader Complexity, use the Left Mouse Button to click on the View Mode option at the top of the Editor Viewport. Currently the View Mode is set to Lit the default.
Once you click on the View Mode with the Left Mouse Button, a menu will appear. From the menu, select the Shader Complexity option.
After clicking on the Shader Complexity option, the level viewport should look like the following.
From this view you can tell where possible performance issues could be happening.
The areas that are in red are becoming very expensive to render while areas that you see in green are cheaper to render.
While it is not 100 percent possible or advisable to get rid of all overdraw, this view mode can make it very helpful when trying
to figure out exactly where your performance issues could be coming from.