Well, she's not actually slow compared to other girls her age, she's just slow compared to other girl soccer players her age. It affects her self confidence when she plays to the point where she concentrates more on running than on her skills. Playing defense, she has the added stress that every time she's outpaced by a forward, there's a chance of a goal.
Which brings me to this summer's training. Her team practices are all about skills and your speed is whatever God gave you. She's going to need to work on speed on her own. She'll never be a world-class sprinter, but she can certainly get faster through strength and technique. As her dad, I will need to set expectations for her. What should those expectations be?
It's all about σ, the standard deviation.
For the 40 yard dash, a good benchmark for a defender, she's probably right around the 50th percentile of all girls. Since we know it's unreasonable to expect her to run as fast as the fastest girls, it's pointless for me to look up record times in the 40 yard dash and use those as a marker for her performance. Instead, I looked up σ, the standard deviation for high school girl student athletes. It turns out that for the 40 yard dash, σ=0.37 or thereabouts. Just as a reference point, the mean is about 6.05 seconds.
The standard deviation of all girls her age gives me a guide for what I can give her as my expectations for her training. Since she will be doing much of her training on her own, these expectations will spur her on to perform*. If her summer training improved her 40 yard dash time by just 0.4 seconds, she would jump from the 50th percentile to above the 85th. It's probably unreasonable to expect anything more than that.
* - If you think this is harsh, then I'm sorry. Real life has expectations, too and they aren't given to you by a loving father. Instead, failing the expectations of life can lead to job loss, divorce or worse.