Is manual driving faster than Ignis driving?

No, there's no difference in speed, but manually driving leaves room for error when you need to make turns, which could affect your travel time. Ignis will make necessary turns automatically.

I've seen one case where manually driving is advantageous: if there's an Imperial dropship on the road (but not so close that you can't get in the car), and you'd rather avoid it than fight it, Ignis won't navigate around it even if he could. He'll plot the shortest route to the destination and drive the car right up to the enemies, and then you'll be committed to the fight. Driving manually, you can take a longer route that avoids the battle, possibly saving time overall.