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This must be something to do with the way they've set the transitions up (i.e. they've built in some delay). I've had no performance problems with d3. Do you mean when the tooltip fades in or something else?

Yes, it is the transitions of the tooltips, the circles that appear when you hover over a datapoint and the colour changes.

All of these stutter sometimes, especially if you are moving your mouse to a point and happen to trigger a few other events on the way.

Firefox 13.0, Kubuntu 10.04 Intel Xeon CPU X3450 @ 2.67GHz 4 core 16GB RAM

Chrome (and any webkit) performs pretty good IMO. Working on getting Firefox and IE9 up to speed. I will be allowing transitions to be disabled, which should help out a bit.

Which browser are you using? I would suggest trying Chrome.