The comparison that I was aiming to make was comparing a "busy wait blocking" implementation, e.g. sending one byte at a time, until the byte has been sent, whi

The comparison that I was aiming to make was comparing a "busy wait blocking" implementation, e.g. sending one byte at a time, until the byte has been sent, which is typical for bare metal embedded systems (that don't use some kind of DMA or concurrency like threads or tasks). In this case, your CPU would be left spinning for an equivalent amount of time to the line rate of the serial port, if you were busy waiting for data.