Is market price real-time?
If you signed the exchange agreement, then the market price is presented in real-time. If you did not sign the exchange agreement, then you have been defaulted to delayed quotes, which are delayed up to 20 minutes.
All other portfolio holdings are valued using the previous day’s close.
Buying power is updated continually throughout the day on a real-time basis. It’s always subject to the maximum margin limit on the account.