Here in my FF, they are smooth, but they make it feel very slow (at least I think it's the animations that cause this impression). I hope they are configurable, saw nothing in the readme, but I probably just missed it.

Odd. Here on Firefox/Linux using an underpowered laptop and 1080p monitor (indeed not the most optimal set up, but it's what I got right now), I expect most animations on modern websites to be janky.

But Sortable is completely smooth for me, yet Dragula lags or stutters a little.

I really wonder what the difference is?

I do have to say, even though they were slower, Dragula's drag-n-drop felt more "solid", in a way. Though I would guess that's some subtle clever placement tweaks, nothing that should affect performance.

The sortable animations are incredibly bad on ipad 2 (ios 7) as well.