Factors aside from grasshoppers and hardening off aside, the answer to your core question is that the speed of growth depends upon just a few basic things: light, temperature, moisture, and food. If you have two plants of the same variety and start (and keep) them one inside and one outside these four factors will make the greatest impact on them. More light = more growth. Warmer temperatures = more growth. Appropriate moisture = more growth. Appropriate food = more growth.
As noted above, outdoors you are going to potentially have more temperature swings, but you're also going to have more light available (unless you plant them in a spot that gets shade.) Moisture and food, for the most part, can be controlled by the grower, regardless of environment.