UART speed

Hi
I have red those posts about UART CSR speed and now I'm trying to do some speed translator. I'm connected to BT UART with 921600 and at PC's side it's 115200 but it didn't work so I can't understand why it is happening like this. I also tierd to connect through 2 USB dongles and it's the same - about 5kBps max. Ofcourse I'm talking about RFCOMM - RS232. Where is the rest of speed???
I don't understand this. Can anyone explain this to me??

Regards

Maciej