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There is not natively support for per-object drawing speed variations, but you can probably work around that by using the layer-based controls. You can find documentation for them here: http://wiki.evilmadscientist.com/AxiDraw_Layer_Control
To use this method, select all of the objects that you would like to plot at the same speed, and move them to a layer that is set to print at a given speed.
Alternately, if you're coding this, feel free to give each and every line segment its own layer. However, be aware that there will always be a pen-lift operation when switching between layers.
If consistently controlled line width is your goal with different speeds, you'll likely also want to use the constant-speed method, as otherwise there will be acceleration on pen-down strokes.
If you want to vary the speed within a stroke, then yes, you'll need to use a low-level interface of one type or another. You can either use the direct serial interface (EBB commands), the robopaint API, or other means to do so. If you can say what tools or environment you are using to generate your vectors, I might be more able to point you in the right directions.