@Darby-Cox this is a common issue when using external drivers. Such drivers have a limit to the speed at which they accept pulses. You have the M569 T parameters set at 2.5 microseconds high time and 2.5 microseconds low time, so the maximum pulse rate is 200kHz. You also have a high steps/mm of 320. So the maximum rate at which the belt can move is 200000/320 which is 625 mm/sec. On a CoreXY machine this will be the maximum diagonal speed. The maximum speeds of X and Y moves will be double this.
To get higher speeds you will either need to reduce the M569 T parameters if your servo drives can handle a higher pulse rate, or reduce the steps/mm by reducing the configured steps/rev of your servo drives or by using larger radius motor pulleys.