I trade FX with Interactive Brokers every day.
The liquidity on EUR/USD is around 50 Million on each side practically all the time.
If you want to have instant fills at the best price available, you have to use limit orders,
and you will have to use BookTrader. FXTrader is not good enough.
In BookTrader in order to have guaranteed instant fills with limit orders, you have to click ANYWHERE ABOVE the ASK Price within the BID SIZE column for a long order. It will fill you at the best ASK price, instead of where you clicked.
Do the opposite for short orders.
The reason is, that in fast moving markets if you click exactly on the ASK/BID price, it can happen, that the price had already moved on. By definition, with limit orders you can only get the same or better price than what you clicked. So if the price has already moved on in the same direction, clicking on the ask you will end up having effectively clicked below it, and your order might not get filled.
This sounds complicated, but it is not in practice. Just try BookTrader.