Really? I didn't know that. I'll admit I know very little about spot FX, but my understanding was that there were seven or so banks that provided a lot of liquidity in FX. Wouldn't they want to have an idea what other spot venues are quoting to prevent someone from arbing them? I would believe you if you told me that a vast majority of the interbank FX trading took place in the forward market or off exchange though.
In FX you typically have the option to turn off trading with counter-parties that you don't want. They wouldn't be able to even see your quotes. Then, a lot of people have "last look" which is pretty much what it sounds like. If someone thinks you are arbing them against other venues you will soon find that you have far fewer people to trade with.
It's a strange game that's very relationship driven. It's all beside the point. No bank system I've ever seen has ever been fast at doing pretty much anything. There's really no reason to be.