Same experience--PDF rendering was too slow even for a lot of linear reading. I remember once opening a statistical text (Springer has a lot of freely downloadable content if you're at a university). I think it was a Bayesian text with a plot showing model convergence. Because the graphic was a vector plot containing lots of line segments, the page would take several minutes to render.

I loved the screen though. I'd love to have a DX sized reader with even the processing power of a raspberry pi. Actually, I'd love something with a keyboard cover that let me do emacs in the sunshine, but that's an altogether different story.