large transaction cube performance

Hi,

I m having a huge volume of 400million records in my transaction cube . Can any one claryfy me how its going to impact on the bps performance while adding new plan data records to my transaction cube.

in sandbox i prototyped to see that while adding new records to the planning cube bw system is reading data records from the planning cube with the same key even from the closed request and ads the differences of values to the planning cube but not the actual value what i had planned.

how can i improve the bps performance .

Thanks,

Ramkumar