Latex documents are typically slow on PDFjs because it positions characters individually. I think it's because Latex has some specific formatting that the PDF spec/apps can't handle well enough. So for text selection PDFjs wraps each character with it's own absolutely positioned element. The massive number of DOM elements cause browsers to slow down to 5-6fps.

I've been working on coalescing the elements and you can see the fruits of my labour by dropping a PDF into https://web.notablepdf.com which is an annotation app based on PDF.js.