[geordief (OP)]

I am guessing that we can't and that "speed" comes into the equation when the method of effecting change or exchanging information   is taken into account.

Causality itself seems to me to be perhaps  more fundamental than the maximum speed limit in a vacuum and perhaps all that can be said (well by me) is that it goes in one direction ?(even that with apparent caveats since there may be a non zero possibility that it may not on occasion)

(Is there even a non zero possibility that it doesn't even move at all on occasion? )

edit the part  I have underlined in red  I am on reflection very unsure of  and think that I may just have misremembered it.

Perhaps all I have heard is that the direction of time  may be immaterial at the quantum level         

[yor_on]

I agree, macroscopically causality is a one way arrow with what I know. When it comes to a quantum regime it becomes discussable. Maybe you can connect it to decoherence although that doesn't tell us anything really, well, more than we define a border for it.
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you might also think of it as 'time' having no predefined direction at a quantum regime but when scaled up becoming a 'clock'? That would be a causality too in a way if we define it so that no matter the arrows direction our 'observer' always find it to go from cause to effect. Like a billiard table where you can play the movie backwards still finding cause and effect to exist. In this case a 'one way arrow / clock' becomes something different from the way we usually think of it with causality becoming what defines it.
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And to answer your question of a speed. For me it has to be 'c', as a limit, no matter its temporal direction. Unless we discuss entanglements.