Even on slow devices the only problem I have had with PDFs has been when they are rendered using the JS renderer. The developer of pdf.js replied to my comment

Even on slow devices the only problem I have had with PDFs has been when they are rendered using the JS renderer. The developer of pdf.js replied to my comments on performance somewhere once, and I think it might have been HN, but was quite happy to acknowledge (IIRC) that its not a high performance solution.