Not read all of this but 2 sides to this. Yes the group could have broken into smaller groups and waved the driver on but they were not obliged to do so. Would have been nice and may have placated OP.
Had this been a tractor with a hay trailer on the back or some horses it would have been doing similar speed and you would also not have been able to pass so the OP would have had no option other that to wait until clear to pass.
I have seen artics and tractors etc use pull ins to allow faster moving traffic to pass around some of the highland roads. It is much appreciated. However if they pull in I would not remonstrate with them as to why they did not do so sooner. In the scheme of things it does not take long to safely make any lost time.
Why the rush? Enjoy the scenery and live life a bit more I reckon.