I inject Fiasp before eating.
The rate at which Fiasp reaches its peak varies from person to person. The rate at which carbs are digested varies from person to person and is different for different types of food. The idea is to match the peak of the insulin with the peak of the carb digestion.
For me this means injecting Fiasp 10 to 15 minutes before eating. I believe this is the standard advice.