SCB's FX conversion happens outside the brokerage account. It is immediate settlement within your savings accounts and then you transfer the converted FX to the FX settlement account and trade. So it is immediate. The downside is the wider spreads.
Saxo conversion is also immediate, but the exchange rate is even worse. I dont have much experience with any other brokers. Saxo has this concept of subaccounts for different currencies. If you have only SGD main account then the currency conversion happens automatically while buying the foreign currency stocks, but you get horrible exchange rates. You can open USD, GBP, EUR etc subaccounts and then do the currency conversion, you get slightly better rates but still worse than SCB or you have the option of directly wire transfering the FX to the subaccount.
TDA, doesnt understand any currency other than USD, the conversion happens outside the brokerage account.
IBKR, I find converting to a margin account solves all these issues.