Max data transfer size/rate via USART?

Just to add: on the receive side, you can check E.getErrorFlags() to see if the receive buffer has overflowed (which happens if your on('data',.. hander can't process characters fast enough (at the 512 byte input buffer gets full) - but for what you're doing that's super unlikely - I'd only expect potential problems if you go far above 115200 baud or do some hugely long calculation in the data handler.