Crystal Reports performance is too slow

Dear SDNers,

I have designed a crystal report which is fetching data from a Z developed function module. From crystal report side, used filters.But while executing report, it is calling the function module multiple times.

due to this performance is very bad. Why report is calling mutiple times of the function module. Do need to modify from crystal report side or from function module side. Please clarify.

Regards,

Venkat