I have Sony's Digital Paper. It is beyond awesome. I notice no lag while writing, and yes it does feel like writing on paper. Is there some way maybe you can measure the difference in what you are observing, lag-wise? Lag has never been an issue I even perceive with Digital Paper. It 'just works'.
I had the DPT too and the remarkable 1 was noticeably better, and the 2 is noticeably better than the 1. I put some gifs in this article but it's really easier to feel than see.
generally the approach is to point a camera at it and record yourself under real world conditions writing and drawing. For instance, if a fast stroke starts to show up by the time you're already done drawing it, that's problematic latency, even if you personally don't notice.
