Many cards charge 1%–3%; plenty of travel cards are 0%. Bring a no-fee card.
UnionPay usually waives the CNY conversion fee; also notify your issuer so travel spend isn't blocked by fraud controls.
Look up the destination currency's mid-market rate (search "X to Y"). Compare it while paying to instantly catch DCC markups.
Related: cheapest way to spend abroad · ATM traps abroad.