Hi this is venkatesh,
this is my answer.
if it is too big means that is submitted into the 
background processing.
that is which is using the standard program RSBDCSUB.
that is done by go thorugh transaction code is sm35.

atually all large programs are done by the background 
porcessing  only.

use performance tuning steps to get good performance of ur 
program.

If the report is taking time to fetch data from the database
server we have to make sure that there is an optimized
"query". In order to have an optimized query we need to
write an optimized "where" conditions. That's database part.

Coming to the Programming part. 

Say example. Our program has the usage of 95% and the
database usage of 5 % then we have to see the program's
performance. Try to reduce the number of LOOP's in a program
and also try to use the logical operators where ever
necessary to reduce the size of the program . that could
solve the problem.

Performance of reports could be increased by avaioind nested
select statements. Instead use Select ... FOR ALL ENTRIES.

Restrict the use of Select... Endselect, instead go for
select .. inot table itab and then loop at itab.

Avoid nested loop statement, Do not use select within loop.

Do not call function module within loop statement.