I totally agree with you, Shaggy (my gh) usually run at half speed to avoid other dogs getting discourage from chasing him. But I've also seen him getting caught by a jack russell (a pretty fast one anyway).
Something that draws my attention is that Shaggy doesn't go double suspension (if he does, is very subtle)... I think it might be because he grew up playing at the dog park with an irregular terrain, changing directions constantly, etc... not sure if that affects the way he run... maybe he start doing it later on, he's first birthday is this monday