Transmission through serial port hangs with high Baud Rate

Hello, I’m using Fez Spider board with USB-Serial Module. I’m getting upload speed about 23 kB/s, using 460800 Baudrate and 5kB packets. When i try to transmit larger packet, about 1/3 - 1/2 of the bits is lost during the transmission. Using smaller Baudrate I can transfer larger packets, but it decreases speed. I tried 921600Bauds but transmission hangs, except if the packet length is smaller than ~40 bytes. I’d like to get speed at least 100kB/s speed. Is that possible? I made a rework on the USB module to enable RTS and CTS but it doesn’t help. Serial port is configured as follows:
On the Spider’s side:

_serialPort.Configure(460800, Gadgeteer.Interfaces.Serial.SerialParity.None,
                        Gadgeteer.Interfaces.Serial.SerialStopBits.One, 8);

where _serialPort is a UsbSerial type

On the PC’s side:

I tried different configurations but this one is the fastest.