Nice interview. Some of these guys memories might be, understandably, getting a little fuzzy. Nurmix makes a good point about game speed; the frame rate seems different for all five speeds. Compare it with Dungeons and Dragons; where all four levels are the same speed (background scrolling) but the objects move slower. Or other games that have five or more levels (eg Space Battle, Star Strike, Sub Hunt) where levels four and five are the same speed but the game is harder in other ways.
Space Spartans is a little different for speed selection compared to the other older games, it's selected after you pass the title screen. I was looking at the Exec documentation and it describes two global variables in the exec. One variable controls the game speed by adding "interrupts" to the clock cycle. Three is the standard number of interrupts, and pressing keypad 'one' at the title screen adds one interrupt, pressing keypad 'two' makes it five interrupts, and pressing keypad 'three' is the slowest at six interrupts. I think this corresponds to 20Hz/15Hz/12Hz/10Hz frame rates. Even though Dungeons and Dragons uses the standard Exec method for selecting its four levels they must be overriding this game speed variable to always be three. Somehow Space Spartans has five speeds. A value of one or two might correspond to 60Hz and 30Hz frame rate but the Exec instructions say not to use those values. Then there's a second global variable that affects objects speeds (combined with each object's velocity). I'm guessing AD&D uses this one for its different levels.
When I got a new game I always started playing at disc speed. I think this is because the early sports games were just too slow at any other speed. If the game was too hard, eg AD&D, I would try one lower. I always thought Space Battle was a little odd having five levels; and then pressing keypad four gives you level five. Sub Hunt is a little odd as well since it looks like the disc is level three rather than level four. I'm not considering the third party games here.