If you trade couple hundred million US dollars a month, you can get pretty decent spreads and fills at forex prime brokers.
They are deep liquid providers. Fill is almost immediate. They can fill a $20 million order in couple seconds like it is nothing. Slippery is rare. In addition, they will give you volume rebate to offset the minimal spreads you pay.
What is the minimum you need to open an account at a prime? Last I heard it's gone up to > 5M