Liquidity
A liquid market has many buyers and sellers and tight spreads; large orders can be filled without slipping the price. EURUSD during the London/New York overlap is the most liquid spot market on earth. Exotic pairs at Asian-session opens are far less liquid: spreads widen, slippage rises, and stop runs become more common.
Liquidity follows the trading day: London open, NY open, and major news releases bring the most activity. End of New York to early Tokyo is the thinnest window.