I'd almost think that high speed is more
necessary in turn based mode. It affects how quickly a character is ready to take another action regardless, and in turn based mode you can't do things like manually dodging attacks.
If you're in real-time, you're probably either going to overwhelm your opponent regardless or you're taking advantage of the increased mobility it offers even though enemies take more shots.
So what's a good starting number for speed? I can't tell how much I need.
I assume you are asking for MM6, 7, and/or 8, since you referenced Turn based.
One thing to know is that certain values in a stat are breakpoints that give you the next boost. Anything between those values doesn't help you. For example, 50 and 75 are two break points, so 50 is as good as any value from 51-74 when considering the bonus. As you get higher values, the breakpoints get farther apart. After you get to about 50, you start needing 25 points to reach the next boost.
Early on, aim for at least 13 in each relevant stat (ex. Knights don't need any Intellect or Personality) to avoid penalties. From there, prioritize stat boosts by what the character does for you.
Here's a chart with stat values and the boost they provide: https://grayface.github.io/mm/mechanics/#Statistics
If you're talking about character creation, then my answer varies somewhat based on which game you're playing. MM7, I wouldn't invest your starting points in Speed on a Dwarf (regardless of their Speed value) because it is inefficient. You'll find barrels of Purple liquid and equipment that can handle the difference much more efficiently.