I wanted to add re: reaction fire, that indeed the speed rating affects reaction speed (speed attribute seems to be used for both TUs and rxns). In xcom, the remaining TUs at the end of turn also affected rxn speed (more TUs left, faster rxn speed). Not sure how the mechanics work for UFO:AI though.
Also, one quirk worth mentioning is that I've had agents take rxn shots during civilian movement, just like how the aliens can take rxn shots at your agents in range even if the shooter isn't targeting the active agent that is moving/performing an action (of course in this case I mention it isn't even the aliens' turn).