Peter,

I've done some timings of serial.receive() in the past.
For me, each round-trip to poll for received data takes approximately 40 milliseconds (you should add your own logging to test the timing). It is the nature of the beast. Also, serial.send/match is explicit and sends string every time. Whereas, a repeating one-way 232 action, I suspect,(you'll need to do testing with Wireshark) sends the first string down and then for repeat, says repeat last string sent (latch repeat) until I tell you not to do so. Then, when button is released, the latch repeat is cancelled.

Your advanced algorithm is likley the only way to make the ramp on RFX as fast as it is on the PC.