Rob,
You are right. Sorry for overlooking the 120 resistor and the slope mode. According to the specs, the high-speed mode of operation is selected by connecting pin 8 to ground or lowering of V(Rs) to 0.0v < 1.0v (p.2,3). So, no high-speed. I believe the manufacturer sets the board to 'slope' as a typical mode that helps to improve EMC behavior (spikes, harmonics) in case we use an unshielded bus cable in order to save some money, but, the trade-off is a loop propagation delay around 100ns with 10K (p.22,23). Anyway, you may reduce the baud-rate to 250 or 500K if necessary. regards.