Only deque from the USBtoSerial's USB->USART buffer when the USART is idle.

LUFA/DoxygenPages/ChangeLog.txt

@@ -22,6 +22,7 @@
   *   - Fixed missing HID report ID prefix on HID class driver GetReport request responses (thanks to Bert van Hall)
   *  - Library Applications:
   *   - Fixed spurious 0xFE USART byte sent in the USBtoSerial project when the baud rate is changed (thanks to Carl Kjeldsen)
+  *   - Fixed blocking USART reads causing low throughput on slow baud rates in the USBtoSerial project (thanks to Nevada Smith)
   *
   *  \section Sec_ChangeLog140302 Version 140302
   *  <b>New:</b>

Projects/USBtoSerial/USBtoSerial.c

@@ -133,8 +133,8 @@ int main(void)
 			}
 		}
 
-		/* Load the next byte from the USART transmit buffer into the USART */
+		/* Load the next byte from the USART transmit buffer into the USART if transmit buffer space is available */
-		if (!(RingBuffer_IsEmpty(&USBtoUSART_Buffer)))
+		if (Serial_IsSendReady() && !(RingBuffer_IsEmpty(&USBtoUSART_Buffer)))
 		  Serial_SendByte(RingBuffer_Remove(&USBtoUSART_Buffer));
 
 		CDC_Device_USBTask(&VirtualSerial_CDC_Interface);