Sometimes you might notice that a particular imported PowerPoint file (or PDF file created from a PowerPoint file) is rendered slowly when scrolling through the pages leaving sections of the page white. It may look similar to this:
It's an iOS PDF rendering issue that GoodNotes has little control over. While the Files app can render the original PowerPoint file smoothly, if you "print" it to a PDF from within that app, the app will also have difficulty displaying the PDF.
Please convert the PowerPoint file to a PDF with an online tool like Zamzar.com and import that PDF instead.