Level Design in UE4 is slow and tedious.

That’s subjective. Whatever, “look” you are going for really has nothing to do with the necessity of blocking out levels for certain game genres. The approaches of the tools outlined above by those asking for more are archaic, but archaic can come with a speed benefit.

Anyway, the best approach for rapidly blocking out levels in UE4 is to make simple lego style meshes in your 3d modelling package of choice and create your own procedural system(s) in Blueprint:

Epic Blueprint Procedural Tutorial
Procedural Building Generation 1
Procedural Building Generation 2

I can see the need/want of better modeling tools to alter BSP geometry in more of a modeler fashion, but at the end of the day Blender is free and anyone can make cubes, planes, cylinders etc - import them into the engine and have a go at creating their own procedural workflow. There are enough static meshes in the starter content to practice with. With spline based deformation coming in 4.3 many, many new avenues will be opened up in terms of procedural generated content.