[awy]

"significant" has a more concrete meaning when understood as part of an optimization problem together with rolling resistance etc tradeoff considerations. op's statement can be transformed into "speed at which aero becomes significant factor in vehicle design."

take a bicycle for instance, there's often tradeoff between rolling resistance/weight and aero. it's why climbing bikes have a different design compared to time trialing bikes. significance is understood as relative to other design considerations.