Some PDF files are complex vector-based PDFs with many layers, and the system needs to process the entire document in order to display a single page. Even a simple PDF transcript can exhibit this behavior, and it can be an issue even on the latest iPad or Mac devices. For example, when a relatively simple 300 page transcript is selected there may be a delay displaying the first page, or when you scroll through a number of documents it will stutter, or slow down the scrolling as you pass these documents.
This issue can be fixed by converting any problematic PDF file(s) to be Linearized PDFs. The benefits of linearization for PDFs is used mainly for the web where an app only needs to load what is being displayed. For example, if you have a 500 page document, linearization will only render the fist few pages that you need to be able to view right away, as opposed to waiting for all 500 pages to be rendered before showing you page 1.
In order to linearize your PDF you should open it in the Preview app on the Mac, then choose File > Save As (hold down the Option key as you click on the File menu to see the Save As option) and check the box for Create Linearized PDF in the resulting dialog. This process of creating linearized PDFs will create PDFs which will perform much better in many applications.
If you’re on Windows you can Google “create linearized PDF” to find a solution to convert a PDF to a linearized PDF.