On the museum layout, I run prototypical speeds, which is to say around 35-45 scale mph for ordinary freight, 50-60 for reefer blocks, and 60-65 for passenger trains. I will run a fast streamliner like the Hiawatha at up to 100+ scale mph, just like the prototype. This is on a large club/museum layout with long straights and wide curves. At home I run much slower because my layout is small. Total size is 13x13 feet, so my straights are only 5-6 feet long. I seldom go over 40 or 50 scale mph, although I do have 072 curves on the outer main line.
DCS allows you to set a maximum scale speed for each locomotive. I sometimes use this feature to keep me from running freight engines too fast. I have my UP 4-12-2 set for 45 scale mph, which is the speed the railroad told the engineers not to exceed (this was later increased when the engines were rebuilt). I don't bother with the speed limiter on the Hiawatha, since the prototype was known to hit 120.