It is not the size of a PDF file that matters in this case, but rather, the complexity of page contents.
Many engineering or architectural diagrams, especially out of CAD or circuit design programs consist of a tremendous number of individual line segments and vector shapes. One of the factors that we have seen that can aggregate performance is that the PDF generating software applies PDF transparency to objects for which transparency is totally unnecessary. You can determine whether this is the case if you open the PDF file in Acrobat Pro DC on a Windows or MacOS computer and use the available tools to analyze the characteristics of those objects.
If you want to make a sample file available, we could do a quick analysis of same for you.