Canvas shadowBlur is much slower than its browser implementation

I've tried the Qml implementation (attached, by the qmlbook.org author) of the W3C "pretty glow lines" 2d canvas example [1] enabling the shadowBlur and setting it to 10 as it was in the JS glow lines implementation.

However, while the javascript example runs fast and smooth on all the devices I've tried and with all the browsers (that supports canvas), the Qml version starts hanging as soon as the "context.shadowBlur = 10;" line is enabled.

For some reason this does't seem to use any acceleration, and also if the visual result is similar (in terms of quality), the speed is way different.

I think this is quite a major issue, as currently the shadowBlur is not usable in qml canvas.

[1] http://www.w3.org/TR/2dcontext/#examples