I assume that you mean the maximum throughput that can be achieved.
The host processor must be able to drive the UART with a baud rate of 921600 and handle the UART transmission efficiently. It must be able to deliver and/or process the data fast enough.
If that is guaranteed, a realistical data throughput of 55 - 60 kB/s (~ 480kBit/s) can be achieved.
Please note that the remote device must be taken into account as well and can be a bottleneck, too.