I think both can be a problem.
It is relatively easy to test: save your project under a new name and in that new file delete all DWGs and see if the speed improves. If it does then the DWGs are the reason.
Or delete all the PDFs and see if the speed improves. If it does then the DWGs are the reason.
If you delete all DWGs and PDFs (placed bitmap image files may also be a source of slowness) and the file is still slow, then probably the size of the model is the cause.
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