Export 3D model to a game or FPS-style navigator

Hello, I am an architect who uses SketchUp, Revit and AutoCAD to create 3D models. I am very frustrated by how difficult it is to inspect these models, and get a feeling of how it will look. Sketchup walthrough is great, but very slow and laggy with big models. The other programs are hopeless, I don't want to export an animation, but simply to walk around the model/landscape like I do in Battlefield, Mount&Blade or any other game.

Why has no 3D modelling program managed to make a walkthrough engine as intuitive, fast and useful as the simplest FPS game? And more importantly, is there any software or game that lets me import my 3D model to walk around in? Maybe some open-source game that allows imported models in CAD or 3D object format? I simply want to look around using WASD control and mouse to look around, I don't care if it means a gun and crosshair must be included.

There are a lot of engines that can do this kind of work, what you need is a realtime engine...but normally they are not so easy to use since they're conceived for game development.
If you want a open source solution Blender Game Engine is the answer.
If you want a easy solution you should take a look to Lumion or Twinmotion that are dedicated to the archiviz

Thanks a lot! I am disappointed to hear that game designers are expected to deal with less easy to use programs than architects.

[Fletch]

As an expert in all programs you mentioned, cad, revit, sketchup, I can say that I, too, am blown away with how absolutely archaic Revit and CAD are for looking at a model... which is why I've resolved to never give them another dime of my money. (I'm a registered architect and designer)
SketchUp's the only one that is close to being "ok" for this, make sure you are using the latest version, and play with the openGL settings, and make sure you have a good video card.