There's a few factors. As you mentioned, the number of people charging at once is definitely one of them. The station as a whole can only provide so much power, so if enough people are charging at once then it'll start to slow down. Another big factor is how much charge your car is at and what year/model you have. The older models don't charge as quickly at higher battery percentages.
Another factor I will add is the temperature of your battery. If you charge at night after driving home from work, it will charge faster than if you pulled it out of the garage in the morning after sitting all night.
Good question, and the only other factor I'd add to the previous answer is the fuller your battery is when you begin to charge, the slower it will charge. So your starting at 130, is slower than if you started at 5, or faster than if you started at 230, for example. The analogy I like is when filling a glass with water; as you approach the top of the glass, you pour slower.
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