1. Here's why
If the model you are importing is not very complex and Lumion is very slow after you place the model in a Scene, it might be caused by an issue in SketchUp.
Some SketchUp models that have curved surfaces, such as a sphere or bottle, might use a so-called projected texture mapping method which is not supported in other applications.
The result is that the model will become very slow to display and render in Lumion 8.0 (and older versions).
Lumion 8.3 and newer are not affected.
2. Verify if this is what's causing the problem
2.1: You can verify if this is what's causing the problem by exporting the .SKP file to .DAE format in SketchUp:
File menu -> Export -> 3D Model -> Save as type -> COLLADA File (*.dae)
2.2: Options... -> Export Texture Maps -> On
2.3: Then take a look at the textures folder that's created in the same location as the .DAE file.
If there are hundreds of tiny cropped textures called SameMaterialName_Number it means that the objects that are using this texture are using the unsupported projected texture mapping method (this often happens when the objects have been imported from a different application).
The screenshot below shows an example of 1100 tiny textures from 1 object.
This results in 1100 different Imported Materials in Lumion, and the number of different materials in a Scene has a huge impact on the framerate.
3. How to resolve the problem the easy way
3.1: Once you have identified the model/material that is causing the problem based on the .DAE export in section 2 above, simply assign a different material to the problematic part of the model in SketchUp and re-import it in Lumion.
4. How to resolve the problem the hard way
If you can't assign a different material to the model that's causing the problem, there is another way - but it won't work for all models:
4.1: SketchUp Window menu -> Extension Warehouse -> Type in "SketchUV" and install the plug-in.
4.2: SketchUp Extensions menu -> SketchUV -> SketchUV
4.3: Double-click on one of the problematic objects, and keep double-clicking until you can select the faces of the object. Press CTRL + A to select all of the faces of the object.
4.4: Click on the SketchUV Mapping Tools button in the tiny SketchUV window.
4.5: Right-click on the selected faces and select Triangulate.
4.6: Click on the SketchUV Mapping Tools in the tiny SketchUV window, and select the faces again.
4.7: Right-click on the selected faces and select Save UVs.
4.8: Right-click on the selected faces and select Load UVs.
4.9: Repeat step 4.3 to 4.8 for other problematic surfaces.
4.10: Do a test export to .DAE format to see how many textures SketchUp is exporting. If the number is decent, i.e. below 100, proceed to step 11.
4.11: Save the .SKP file under a new name and re-import it in Lumion.