Enforcing requirements so XON/XOFF software handshake works and no drops of data are experienced while transferring data to an SD Card. I tested this with a CH340 USB to serial adapter (quite typical) at baudrates as high as 1000000 on a Atmega2560 running at 16Mhz, with no problems at all. We need a 1024 byte RX buffer. Less than that and we will have drops, higher than that is not needed

Marlin/MarlinSerial.h

   #ifndef RX_BUFFER_SIZE
     #define RX_BUFFER_SIZE 128
   #endif
+  #if ENABLED(SERIAL_XON_XOFF) && RX_BUFFER_SIZE < 1024
+	#error "XON/XOFF requires a 1024 or greater RX_BUFFER_SIZE for allowing reliable transfers without drops"
+  #endif
   #ifndef TX_BUFFER_SIZE
     #define TX_BUFFER_SIZE 32
   #endif