I did some science on this tonight when I should be sleeping, because it has made me curious. I genned a micro world, started a human male demigod adventurer, as a member of the only human civilization, fiddled with his stats, wrote down his speed, then force quitted and repeated. I also found that starting equipment doesn't seem to make an difference in the reported speed of a character. Their speed value didn't change at all when I stripped and dumped all my experimental adventurers' items.
I started with a demigod with all "average" stats for a baseline. You can see that there is a pretty wide variation(numbers below), nearly 300 points between the highest and lowest scores.
I then separately tested strength and agility at "superior", and they both averaged out at about the same increase, but strength's lowest and highest score varied much more than agility's. This is quite likely due to the small sample set. With only fifteen data points is easy and probable to miss extreme lows and highs.
I then tested both strength and agility together at "superior". Again, similarly to the first baseline test, the spread is a bit over 300 points.
I also tested kinesthetic sense, on the basis that the wiki states that it is important to movement related skills. It doesn't seem to directly impact movement speed for what I can tell.
Here's some numbers.
Human demigod with absolutely no bonus stats and no skills assigned. Ten data points.
Lowest starting speed:897
Highest starting speed:1184
Average starting speed:1008 (rounded to nearest whole number)
Human demigod with only superior strength bonus applied, no skills assigned. Fifteen data points.
Lowest starting speed:1076
Highest starting speed:1572
Average starting speed:1292
Human demigod with only superior agility bonus applied, no skills assigned. Fifteen data points.
Lowest starting speed:1161
Highest starting speed:1398
Average starting speed:1300
Human demigod with only superior kinesthetic sense bonus applied, no skills assigned. Ten data points.
Lowest starting speed:857
Highest starting speed:1168
Average starting speed:985
Human demigod with both superior strength and superior agility bonuses applied, no skills assigned. Fifteen data points.
Lowest starting speed:1692
Highest starting speed:2020
Average starting speed:1818
As you can see, all adventurers do not start equal. I suspect that a character is created with attribute scores that are randomized within the "average" range, and then the player selected bonuses are applied on top of these values. This would account for the high/low spread being about the same for each level of strength/agility bonus applied. The bonus remains constant, while the initial value is what changes.
I made sure to check each new, experimental adventurer's stats screen, and absolutely all of them described the unbonused stats as average. Also, every adventurer was described as "Averaged-sized for a human". I don't think there is a way to have a taller or shorter, broader or thinner character. I take this back. I just happened to generate a character that is "small for a human".