Firmware specifies only the maximum speed. In your case it is set to 10000 (the 4th arument of DEFAULT_MAX_FEEDRATE). 1000 is probably more common. It depends on your extruder. But this value is not really important (it is only the top limit). The actual extruder speed is set by slic3r. The option name is retract_speed. Set it to something low for the tests (e.g. 200).
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