They could use her time/space magic as a viable reason for Ciri being faster. At any rate, it's important she feels different than Geralt gameplay wise.
I'm divided on this. Having her interrupt our story progress with Geralt could be frustrating, especially in replays. 10 quests, if we look at your average quest in previous games, is a lot of game time. We're looking at hours here. Also, will she have access to the open world in any way? That seems unlikely since it will have been designed to react to Geralt ( NPC's mainly), This means sectioned off areas designed just for her parts, and how interactive will those sections be?. On the other hand, I did want to play as Ciri, I just wonder if it wouldn't have been better to play as her in one uninterrupted section, preferably after the game is done.