Their demos performed fine for me on both desktop and mobile. Not sure what the issue is here, you're allowed to require some baseline performance level for something like this.
I had no performance problems, but right on the first demo each scrollbar is broken in it's own unique way.
The main area with the sparkgrid has good horizontal and vertical scrollbars, but they are nearly invisible, being dark grey on slightly lighter dark grey, and only visible if you hover in the area they scroll. Also the vertical scroll bar doesn't take you to the very bottom.
Then there's the top right area, which has a normal looking scrollbar (good), but once you scroll it changes the size of the entire right panel (weird), and if you drag it past the point where it has reached the bottom (really easy to do accidentally) it starts spasming uncontrollably. If you instead use the scroll wheel, you can scroll to the bottom, but if you scroll a bit further you are back on the top? None of the other scrollbars has either of those issues.
Then there's the bottom right, where the scrollbar is visible, takes you all the way to the bottom, and neither spasms nor resets, but just to be special in its own way it starts out as a big bar as if there was little content below the fold, but the further you scroll the smaller the bar gets. As you scroll up, it grows again. It's also stuttering a bit while making these size changes.
Another example besides scroll bars is the lack of tool tips on the UI (they do exist on data though!), which makes for a weird toolbar that moves its content all over the place on hover. And I guess you're supposed to guess that check boxes are delete buttons.
This doesn't take away from the app, it's still cool and useful, but little annoyances like that add up to make it feel weird and un-native (I guess foreign is the word).
It sounds like you have some browser/OS combination with aggressive scroll inertia enabled. A bug report on our GitHub with some of system details would be appreciated!
