The first step I recommend is examining your 3D elements. Your PolyBoard files appear to be quite large; in comparison, I couldn’t find any of my own files exceeding 4.5MB, while the kitchen file you shared is approximately 15MB. This size difference likely stems from using a significant number of 3D elements, many of which are likely substantial in size. I suggest testing the removal of these 3D elements and then, if you’d like to retain them, replacing them with more lightweight alternatives.
This is why we recommend making as many of your 3D elements as possible in PolyBoard, as it doesn’t place as much strain on the program as externally sourced 3D models. If you are importing 3D models, reduce the amount of faces and segments on the models where possible.

We have additional information regarding 3D elements on our help centre, see the following link:

https://wooddesigner.org/help-centre/polyboard-handles-specialist-fittings-3d-accessories/