Even though roast degree has the biggest role on coffee’s flavour profile, total roast time and time of each stage are also important factors. If you roast fast, you will get more desired aroma compounds. But be careful not to burn the beans! Coffee’s total flavour (fruity, berry-like, chocolatey, nutty altogether) is stronger. Also the amount of aroma compounds, which are created in the beginning of development stage, is higher with fast roasting.
In some cases fast roasting is not a good idea. It can be due to roaster design (see next paragraph below) or coffee’s characteristics. Fast roasting enhances all the flavours of the coffee. If we do not want some flavours to the coffee, we need to adjust the roast profile. For example, acidity is normally desired flavour but on espresso blends people sometimes want low acidity. When roasting slower organic acids have more time to break down, the coffee becomes less acidic. This is when slow roasting might be a good idea.