The amount of power you draw from cars is quite limited, so it must be your imagination. The reason is probably because you are doing something instead of going back and forth checking. So it's more your sense of time that's different.
A more detailed explanation:
Typically what you get from a car at it's raw state, is something like 9-12 volts. What you get from your house is around 200-300 Volts, it depends where in the world you live. When you're changing a small gizmo, like iphone/ipad, that has a small power usage, 9-12 volts is just fine. However if you were to charge a laptop you would see what I mean. You would have to convert the 9-12 volts to 200-300, (a factor of 20-30) and would thus have to decrease the amperage with the same factor. The result would be that your laptop could hold power for a longer period of time, but would eventually go flat.
So there is a limit to how much power (Walt) you can draw from a car, and it will always be a lot lower than what you can get at home. If you are a skilled electrician you could raise the amount of power by accessing the car battery directly.
Now why isn't it a lot faster to charge your iphone at home?
Well it easily could be, but charging a battery at a very high pace, will also create a lot of heat. Apple have probably taken this into consideration when they created the charger for the iPhone. A balance between having a low charge time, without heating the battery too much which could risk the health or just the durability of the phone.