I’ve replaced the “Linked Duplicates” of the two basic planks by Empties with the same positions and orientations and hidden the two original planks. Now, 5 planks are remainning which are unlinked copies of the two basic planks. I’ll replace them by empties as I did with the “Linked Duplicates” since the file size is still around 388 MB:

Edit: After replacing the 5 “Unlinked Duplicates” in the screenshot above by “Linked Duplicates”, the file size is now below 112 MB! It takes 16 seconds on my computer to load the file.
Edit2: This is the cleaned-up file here with 111.684 KB:
