The participation settings you choose will have an important effect on the performance of your program.

• Contacts can enter the program anytime even if they are already in it is the least performance-heavy setting. You should use this whenever possible, and exclude the contacts you don't want to enter the program via a filter (based on whatever conversion criteria you are using).
• Contacts can enter this program only once ever has a high performance cost because every contact entering it must be checked to see if they have entered it before. You should use this only if it is absolutely necessary because for a program with a large number of contacts, referencing the program participation table could slow down any other programs using it.