Yeah, drag-scrolling and page-jumping on eink is solidly in the "not great" category.

Maybe you could draw a line showing where [some line of text] is moving to? Like starting from where the finger starts, draw a "timeline" of locations as they move their finger, and when they release you draw the whole page:

    ┌───────────────┐ 
    │               │ 
    │         drag  │ 
    │          ┌┐ │ │ 
    │ │        └┘ │ │ 
    │ └┐          │ │ 
    │  └┐         │ │ 
    │   └┐        │ │ 
    │    │     ┌┐ │ │ 
    │    └─    └┘ ▼ │ 
    │               │ 
    │               │ 
    └───────────────┘ 

(wow, lol, mobile Firefox renders that horribly. I wonder if I'm missing a font...)

You can do small-bounding-box "draw black on top" extremely quickly, it's how reMarkable tablets get their near-20ms pen-to-display latency.

I'd probably also like a "this is where the previous top/bottom/where you dragged is now" temporary underline across the screen at the end, for easier visibility. Just clear it a second later.