I have noticed that CO2 escapes a lot faster than regular air out of my tubes. It's not constant though and I suspect this may have something to do with the tubes as well (I have a batch of light racing tubes that seem to be leaking faster than my heavier tubes but maybe it's skewed perception).

I always pump up my road tires before rides anyway. If I've inflated it with CO2 the day before and it's particularly low when I start pumping, I empty it almost entirely and start again with "fresh air."