This is due to how it has to calculate each pixel and overlapping lights all in one frame. Placeable lights are dynamic lights that have to be rendered in real time. Unreal 4 uses deferred rendering, so you’re able to place down a bunch and get decent performance if they are separate and not overlapping. But if you have several large scale lights overlapping you will have to calculate each overlapping light on each pixel they are overlapping.
If you can avoid overlapping the radii of the lights, then you’ll have less to compute. If you overlap lights, though, then you’ll have a huge cost.
A lot has changed. Unreal 1 used forwarded rendering, which cannot render as many dynamic lights as Unreal 4 can, which uses deferred rendering.