Change speed of TestFX robots?

Question

Specifically the WriteRobot / WriteRobotImpl. It seems to write things rather slowly and I'd like to make it write faster.

Edit
In response to M.S.'s comment I tried this (NB at this point I hadn't worked out that WriteRobot was involved, not TypeRobot):

setup(){
...
    setFinalStatic( org.testfx.robot.impl.TypeRobotImpl.class.getDeclaredField("SLEEP_AFTER_KEY_CODE_IN_MILLIS"), 5 );
}
...
static void setFinalStatic(Field field, Object newValue) throws Exception {
    field.setAccessible(true);
    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
    field.set(null, newValue);
}

Unfortunately it appears to make no difference to the typing speed, even when set to 1 ms.

Edit
I note the comment by Slaw.

I set the System property testfx.robot.write_sleep before running the test: this had no effect, despite one being able to see that it might have, from the source code at the top of WriteRobotImpl.java (see below). When I set this to 500 ms it also had no effect, making me conclude that the property was not being seen by the code there for some reason, so the default 25 ms was being set.

NB possible other causes: following the code there, it appears that WriteRobot.write always results in a call to WriteRobot.typeCharacterInScene, which in turn calls BaseRobot.typeKeyboard and WaitForAsyncUtils.waitForFxEvents. The latter might be a "difficult customer": if each pressed key then has to "wait for events" to bubble up, there may well be nothing to be done about things.

Still trying to work out why the following lines at the top of org.testfx.robot.impl.WriteRobotImpl.java would fail to see the System property:

private static final int SLEEP_AFTER_CHARACTER_IN_MILLIS;

static {
    int writeSleep;
    try {
        writeSleep = Integer.getInteger("testfx.robot.write_sleep", 25);
    }
    catch (NumberFormatException e) {
        System.err.println("\"testfx.robot.write_sleep\" property must be a number but was: \"" +
                System.getProperty("testfx.robot.write_sleep") + "\".\nUsing default of \"25\" milliseconds.");
        e.printStackTrace();
        writeSleep = 25;
    }
    SLEEP_AFTER_CHARACTER_IN_MILLIS = writeSleep;
}

I also wondered whether maybe that static{...} code block happens so early than you need to set the System property before the tests are run. I tried setting this property in gradle.build. Still no success.