Growth rates of the past-game characters are unknown, besides DLC Marth's. Some people assume that they're like an avatar's, due to their max stats being handled in the same way as the avatar's asset and flaw system, but Marth's growths seem to contradict that. They also aren't directly based on his older growths though - in fact, they're all around better, even not counting the +20% growth skill.
Nepheene's max stats are the equivalent of an avatar with asset skill and flaw luck, she starts as a lvl 16 unit with 18 in both speed and defense, and still has her wrath skill, which females usually can't even get in Awakening, aside from inheritance. So, at the very least, it doesn't seem like she'd be slow, aside from maybe the general's class caps, but then she can always be class changed to something else... Although, the only playable on foot classes that can wield lances aside from villagers, knight/general and great lords is the DLC-only Bride (I wonder if they'll rename it to princess or something?).