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.