I have wondered why myself also . Why so fast ?
In my limited experience , the two , 2 cyl Accucraft shays that I had , would operate really , really very slowly , I wish that I had video taped them .
The Roundhouse Forney was a slow operating rod loco , and was so nice that it could be used as a switcher , a really nice loco .
And the 3 Cheddar locos , had to be carefully , so lightly fired , and with only about 10 pounds pressure , then they would trundle along , and operate so very nicely , BUT , at full pressure , the fastest things that I ever saw . The self regulator gas valve that Cheddar later made , was a real help making slow speed operation routine .
They can be operated very slowly , I enjoyed them operating slowly myself .
Sort of like a train show , and all the Lionels are ran so very fast on the display layouts ?